Test uploading images to PRs

[alexr00]

Refs #320

• Windows
• macOS
• Linux

Complexity: 3

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Context

This feature allows users to upload images into GitHub issue and pull request comments within the VS Code GitHub Pull Requests and Issues extension by either using an "upload button" to upload an image file or pasting an image from the clipboard into the comment editor webview and comment widget.

Test Steps

1. Install the latest pre-release of GitHub Pull Requests
2. Open your clone of a GitHub repository
3. From the "Pull Requests" view, open a PR webview
4. Leave a comment from the PR webview. Try both uploading an image using the upload button, and by copying and pasting.
5. Verify that the image uploads and displays.
6. Open a changed file from a PR in the Pull Requests view. Try both uploading an image using the upload button, and by copying and pasting.
7. Verify that the image uploads and displays. There's a known issue where it will show as a broken image for a brief moment.