Requests: Give correct MIME in GetSourceScreenshot

[andrewsoncha]

Description

Adds to GetSourceScreenshot the code to look up the correct MIME type for the requested image format and returns the base64 string with the correct MIME type. The lookup of the correct MIME types for each file formats is done using Qt library's QMimeDatabase Object.

Motivation and Context

This PR solves this issue.
The original code for GetSourceScreenshot got the MIME type by simply appending the requested image format to image/.
In some cases, this causes the response MIME type to be an invalid MIME type.
For Example, when making a GetSourceScreenshot request with the image format parameter of {'imageFormat': 'jpg'}, the original code returns a base64 string with the MIME type image/jpg, which is not an official valid MIME type.
This causes issues in rendering with certain browsers or software.

How Has This Been Tested?

Tested OS(s): macOS 26.02, Windows 11
Made GetSourceScreenshot requests with each supported image format and confirmed that the response is a base64 string starting with the official MIME types
For Example, both requests made with {'imageFormat': 'jpg'} and {'imageFormat': 'jpeg'} respond with the valid MIME type image/jpeg

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the Contributing Guidelines.
• All commit messages are properly formatted and commits squashed where appropriate.
• My code is not on master or a release/* branch.
• The code has been tested.
• I have included updates to all appropriate documentation.