Version

4.15.1

Describe the bug

Same files that used to work perfectly in the Emulator of version 4.14.1, don't get uploaded anymore in 4.15.1 - 400 error code. The bot application's endpoint doesn't even get triggered for such request. Textual messages work fine though.

To Reproduce

Steps to reproduce the behavior:

1. Run Bot Framework Emulator and initiate a conversation with your bot;
2. Click on Attach button to pick a file locally, click "Open" to try to upload it;
3. See error 400 for this POST request

Expected behavior

File should be sent to the bot successfully.

[bug]