diff --git a/services/http.ts b/services/http.ts
index 3e818b4..57791e2 100644
--- a/services/http.ts
+++ b/services/http.ts
@@ -1,3 +1,9 @@
+export type FetchConfig = Omit<RequestInit, 'headers'> & {
+  headers?: Record<string, string>;
+};
+
+export type HttpBody = BodyInit | object;
+
 export class BaseHttpClientError extends Error {
   response: Response;
 
@@ -25,59 +31,66 @@ export abstract class BaseHttpClient {
     return this._baseUrl + rest
   }
 
-  _get(url: string, config?: object): Promise<Response> {
+  _get(url: string, config?: FetchConfig): Promise<Response> {
     return this._send(url, 'GET', undefined, config);
   }
 
-  _post(url: string, body?: any, config?: object): Promise<Response> {
+  _post(url: string, body?: HttpBody, config?: FetchConfig): Promise<Response> {
     return this._send(url, 'POST', body, config);
   }
 
-  _put(url: string, body?: any, config?: object): Promise<Response> {
+  _put(url: string, body?: HttpBody, config?: FetchConfig): Promise<Response> {
     return this._send(url, 'PUT', body, config);
   }
 
-  _patch(url: string, body?: any, config?: object): Promise<Response> {
+  _patch(url: string, body?: HttpBody, config?: FetchConfig): Promise<Response> {
     return this._send(url, 'PATCH', body, config);
   }
 
-  _delete(url: string, config?: object): Promise<Response> {
+  _delete(url: string, config?: FetchConfig): Promise<Response> {
     return this._send(url, 'DELETE', undefined, config);
   }
 
-  async _sendTest(url: string, method: string, body?: any): Promise<Response> {
-    const response = await fetch(this.url(url), {
-      method,
-      body,
-      headers: {
-        'Accept': 'application/text',
-        'Authorization': this._requestHeaders.Authorization
-      }
-    });
-
-    if (!response.ok) {
-      throw new BaseHttpClientError(response);
-    }
-
-    return response;
-  }
-
-  async _send(url: string, method: string, body?: any, config?: object): Promise<Response> {
+  async _send(
+    url: string,
+    method: string,
+    body?: HttpBody,
+    config?: FetchConfig,
+  ): Promise<Response> {
+    const { headers: configHeaders, ...restConfig } = config ?? {};
+    const mergedHeaders: Record<string, string> = {
+      ...this._requestHeaders,
+      ...configHeaders,
+    };
     const requestOptions = {
       method,
-      body,
-      headers: this._requestHeaders,
+      body: undefined as BodyInit | undefined,
+      headers: mergedHeaders,
       signal: this._abortSignal,
-      ...config
+      ...restConfig,
     };
 
-    if (requestOptions.headers['Content-Type'] === 'application/json'
-      && !(body instanceof FormData)
-      && typeof body !== 'undefined'
-      && body !== null
-    ) {
+    // Let the browser set Content-Type (and multipart boundary) for FormData:
+    if (body instanceof FormData) {
+      delete mergedHeaders['Content-Type'];
+      requestOptions.body = body;
+    }
+    else if (body !== null && body !== undefined && (
+      mergedHeaders['Content-Type'] === 'application/json'
+      || (
+        typeof body === 'object'
+        && !(body instanceof Blob)
+        && !(body instanceof ArrayBuffer)
+        && !ArrayBuffer.isView(body)
+        && !(body instanceof URLSearchParams)
+        && !(body instanceof ReadableStream)
+      )
+    )) {
       requestOptions.body = JSON.stringify(body);
     }
+    else {
+      requestOptions.body = body as BodyInit;
+    }
 
     const response = await fetch(this.url(url), requestOptions);
 
diff --git a/services/osm.ts b/services/osm.ts
index 9f41aea..16f18c4 100644
--- a/services/osm.ts
+++ b/services/osm.ts
@@ -1,7 +1,12 @@
 import parseOsmChangeXml from '@osmcha/osmchange-parser';
 import type { FeatureCollection, Point } from 'geojson';
 
-import { BaseHttpClient, BaseHttpClientError } from "~/services/http";
+import {
+  BaseHttpClient,
+  BaseHttpClientError,
+  type FetchConfig,
+  type HttpBody,
+} from '~/services/http';
 import * as xml from '~/util/xml';
 
 import type { ICancelableClient } from '~/services/loading';
@@ -14,7 +19,6 @@ import type {
   OsmElement,
   OsmNode,
   OsmNote,
-  OsmTags,
   OsmWay,
 } from '~/types/osm';
 import type { WorkspaceId } from '~/types/workspaces';
@@ -207,7 +211,9 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
     this.#webUrl = webUrl;
     this.#tdeiClient = tdeiClient;
     this.#setAuthHeader();
-    this._requestHeaders['Accept'] = 'text/plain';
+
+    // OSM API can return XML or JSON based on the header or file extension:
+    this._requestHeaders['Accept'] = '*/*';
     this._requestHeaders['Content-Type'] = 'text/plain';
   }
 
@@ -234,8 +240,8 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
       display_name: this.auth.displayName
     };
 
-    await this._put(`user/${this.auth.subject}`, JSON.stringify(body), {
-      headers: { ...this._requestHeaders, 'Content-Type': 'application/json' }
+    await this._put(`user/${this.auth.subject}`, body, {
+      headers: { 'Content-Type': 'application/json' }
     });
   }
 
@@ -286,7 +292,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
   ): Promise<OsmElement> {
     const response = await this._get(`${type}/${id}/${version}`, {
       headers: {
-        ...this._requestHeaders,
         'Accept': 'application/json',
         'X-Workspace': workspaceId,
       },
@@ -301,7 +306,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
   async getNodes(workspaceId: WorkspaceId, nodeIds: (number | string)[]): Promise<OsmNode[]> {
     const response = await this._get(`nodes?nodes=${nodeIds.join(',')}`, {
       headers: {
-        ...this._requestHeaders,
         'Accept': 'application/json',
         'X-Workspace': workspaceId,
       },
@@ -319,7 +323,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
   async getWays(workspaceId: WorkspaceId, wayIds: (number | string)[]): Promise<OsmWay[]> {
     const response = await this._get(`ways?ways=${wayIds.join(',')}`, {
       headers: {
-        ...this._requestHeaders,
         'Accept': 'application/json',
         'X-Workspace': workspaceId,
       },
@@ -337,7 +340,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
   async getWaysForNode(workspaceId: WorkspaceId, nodeId: number): Promise<OsmElement[]> {
     const response = await this._get(`node/${nodeId}/ways`, {
       headers: {
-        ...this._requestHeaders,
         'Accept': 'application/json',
         'X-Workspace': workspaceId,
       },
@@ -348,7 +350,7 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
 
   async listChangesets(workspaceId: WorkspaceId): Promise<OsmChangeset[]> {
     const response = await this._get(`changesets.json`, {
-      headers: { ...this._requestHeaders, 'X-Workspace': workspaceId },
+      headers: { 'X-Workspace': workspaceId },
     });
 
     const changesets = (await response.json())?.changesets ?? [];
@@ -374,7 +376,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
 
     const response = await this._get(url, {
       headers: {
-        ...this._requestHeaders,
         'Accept': 'application/json',
         'X-Workspace': workspaceId,
       },
@@ -401,7 +402,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
   {
     const response = await this._get(`changeset/${changesetId}/download`, {
       headers: {
-        ...this._requestHeaders,
         'Accept': 'application/xml',
         'X-Workspace': workspaceId,
       },
@@ -427,7 +427,7 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
 
     const body = xml.serialize(doc);
     const response = await this._put('changeset/create', body, {
-      headers: { ...this._requestHeaders, 'X-Workspace': workspaceId },
+      headers: { 'X-Workspace': workspaceId },
     });
 
     return Number(await response.text());
@@ -441,7 +441,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
     await this._post(`changeset/${changesetId}/upload`, changesetXml, {
       headers: {
         'Content-Type': 'application/xml',
-        'Authorization': this._requestHeaders['Authorization'],
         'X-Workspace': workspaceId,
       },
     });
@@ -468,10 +467,7 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
     body.append('text', message);
 
     await this._post(`changeset/${changesetId}/comment`, body, {
-      headers: {
-        'Authorization': this._requestHeaders['Authorization'],
-        'X-Workspace': workspaceId,
-      },
+      headers: { 'X-Workspace': workspaceId },
     });
   }
 
@@ -484,7 +480,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
 
     const response = await this._get(`notes/search.json?${params}`, {
       headers: {
-        ...this._requestHeaders,
         'Accept': 'application/json',
         'X-Workspace': workspaceId,
       },
@@ -497,7 +492,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
     const bboxParam = await this.getExportBbox(workspaceId);
     const response = await this._get(`map.json?bbox=${bboxParam}`, {
       headers: {
-        ...this._requestHeaders,
         'Accept': 'application/json',
         'X-Workspace': workspaceId
       }
@@ -510,7 +504,6 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
     const bboxParam = await this.getExportBbox(workspaceId);
     const response = await this._get(`map?bbox=${bboxParam}`, {
       headers: {
-        ...this._requestHeaders,
         'Accept': 'application/xml',
         'X-Workspace': workspaceId
       }
@@ -525,17 +518,23 @@ export class OsmApiClient extends BaseHttpClient implements ICancelableClient {
     }
   }
 
-  async _send(url: string, method: string, body?: any, config?: object): Promise<Response> {
+  override async _send(
+    url: string,
+    method: string,
+    body?: HttpBody,
+    config?: FetchConfig,
+  ): Promise<Response> {
     try {
       await this.#tdeiClient.tryRefreshAuth();
       this.#setAuthHeader();
 
-      const requestOptions = {
-        credentials: 'include'
-      }
+      const requestOptions: FetchConfig = {
+        credentials: 'include',
+      };
 
       return await super._send(url, method, body, { ...requestOptions, ...config });
-    } catch (e: any) {
+    }
+    catch (e: unknown) {
       if (e instanceof BaseHttpClientError) {
         throw new OsmApiClientError(e.response);
       }
diff --git a/services/tdei.ts b/services/tdei.ts
index 7c3b1e5..4561089 100644
--- a/services/tdei.ts
+++ b/services/tdei.ts
@@ -1,6 +1,11 @@
 import { BlobReader, BlobWriter, ZipReader } from '@zip.js/zip.js';
 
-import { BaseHttpClient, BaseHttpClientError } from '~/services/http';
+import {
+  BaseHttpClient,
+  BaseHttpClientError,
+  type FetchConfig,
+  type HttpBody,
+} from '~/services/http';
 import type { ICancelableClient } from '~/services/loading';
 import type {
   TdeiFeedback,
@@ -237,13 +242,17 @@ export class TdeiClient extends BaseHttpClient implements ICancelableClient {
   }
 
   async downloadOswDataset(tdeiRecordId: string, format: string = 'osw'): Promise<Blob> {
-    const response = await this._sendTest(`osw/${tdeiRecordId}?format=${format}`, 'GET');
+    const response = await this._get(`osw/${tdeiRecordId}?format=${format}`, {
+      headers: { Accept: '*/*' },
+    });
 
     return (await response.blob());
   }
 
   async downloadPathwaysDataset(tdeiDatasetId: string): Promise<Blob> {
-    const response = await this._sendTest(`gtfs-pathways/${tdeiDatasetId}`, 'GET');
+    const response = await this._get(`gtfs-pathways/${tdeiDatasetId}`, {
+      headers: { Accept: '*/*' },
+    });
 
     return (await response.blob());
   }
@@ -284,9 +293,7 @@ export class TdeiClient extends BaseHttpClient implements ICancelableClient {
       resource += '?derived_from_dataset_id=' + tdeiRecordId;
     }
 
-    const response = await this._post(resource, body, {
-      headers: { 'Authorization': this._requestHeaders['Authorization'] }
-    });
+    const response = await this._post(resource, body);
 
     return await response.text();
   }
@@ -308,9 +315,7 @@ export class TdeiClient extends BaseHttpClient implements ICancelableClient {
       resource += '?derived_from_dataset_id=' + tdeiRecordId;
     }
 
-    const response = await this._post(resource, body, {
-      headers: { 'Authorization': this._requestHeaders['Authorization'] }
-    });
+    const response = await this._post(resource, body);
 
     return await response.text();
   }
@@ -327,19 +332,14 @@ export class TdeiClient extends BaseHttpClient implements ICancelableClient {
     const filename = sourceFormat === 'osw' ? 'osw.zip' : 'osm.xml';
     body.append('file', new File([dataset], filename));
 
-    const jobResponse = await this._sendTest('osw/convert', 'POST', body);
+    const jobResponse = await this._post('osw/convert', body);
     const jobId = (await jobResponse.text());
 
     while (true) {
       console.info(`Waiting for dataset conversion job ${jobId}...`);
       await new Promise(resolve => setTimeout(resolve, 4000));
 
-      const statusResponse = await this._get(`jobs?job_id=${jobId}&tdei_project_group_id=${projectGroupId}`, {
-        headers: {
-          'Accept': 'application/text',
-          'Authorization': this._requestHeaders['Authorization']
-        }
-      });
+      const statusResponse = await this._get(`jobs?job_id=${jobId}&tdei_project_group_id=${projectGroupId}`);
       const statusBody = (await statusResponse.json())[0];
       const statusText = statusBody.status.toLowerCase();
 
@@ -352,7 +352,9 @@ export class TdeiClient extends BaseHttpClient implements ICancelableClient {
       }
     }
 
-    const fileResponse = await this._sendTest(`job/download/${jobId}`, 'GET');
+    const fileResponse = await this._get(`job/download/${jobId}`, {
+      headers: { 'Accept': '*/*' },
+    });
 
     return await fileResponse.blob();
   }
@@ -424,14 +426,20 @@ export class TdeiClient extends BaseHttpClient implements ICancelableClient {
     }
   }
 
-  override async _send(url: string, method: string, body?: any, config?: object): Promise<Response> {
+  override async _send(
+    url: string,
+    method: string,
+    body?: HttpBody,
+    config?: FetchConfig,
+  ): Promise<Response> {
     try {
       if (this.#auth.needsRefresh) {
         await this.refreshToken();
       }
 
       return await super._send(url, method, body, config);
-    } catch (e: any) {
+    }
+    catch (e: unknown) {
       if (e instanceof BaseHttpClientError) {
         throw new TdeiClientError(e.response);
       }
@@ -515,13 +523,19 @@ export class TdeiUserClient extends BaseHttpClient implements ICancelableClient
     }
   }
 
-  override async _send(url: string, method: string, body?: any, config?: object): Promise<Response> {
+  override async _send(
+    url: string,
+    method: string,
+    body?: HttpBody,
+    config?: FetchConfig,
+  ): Promise<Response> {
     try {
       await this.#tdeiClient.tryRefreshAuth();
       this.#setAuthHeader();
 
       return await super._send(url, method, body, config);
-    } catch (e: any) {
+    }
+    catch (e: unknown) {
       if (e instanceof BaseHttpClientError) {
         throw new TdeiUserClientError(e.response);
       }
diff --git a/services/workspaces.ts b/services/workspaces.ts
index d9e9709..e9c25e8 100644
--- a/services/workspaces.ts
+++ b/services/workspaces.ts
@@ -1,4 +1,9 @@
-import { BaseHttpClient, BaseHttpClientError } from "~/services/http";
+import {
+  BaseHttpClient,
+  BaseHttpClientError,
+  type FetchConfig,
+  type HttpBody,
+} from '~/services/http';
 import { buildPathwaysCsvArchive } from '~/services/pathways';
 import { compareStringAsc } from '~/util/compare';
 
@@ -238,15 +243,16 @@ export class WorkspacesClient extends BaseHttpClient implements ICancelableClien
   override async _send(
     url: string,
     method: string,
-    body?: any,
-    config?: object
+    body?: HttpBody,
+    config?: FetchConfig,
   ): Promise<Response> {
     try {
       await this.#tdeiClient.tryRefreshAuth();
       this.#setAuthHeader();
 
       return await super._send(url, method, body, config);
-    } catch (e) {
+    }
+    catch (e: unknown) {
       if (e instanceof BaseHttpClientError) {
         throw new WorkspacesClientError(e.response);
       }