Upload refuses in image attachment when canceling from the browser file-selector

Reproduction steps:

1. Go to open a pull request (I'm positive comments, etc. will also cause this)
2. Click on the "image" button to attach an image.
3. Click "Select an Image" in the modal to open the system file selector
4. Select a file, so the file selector closes.
5. Click "Select an Image" again, but this time, cancel out of the dialog box.
6. Hit "Upload".

Expected behavior is that the file should still upload and get attached, even though I've changed my mind about what file I want to attach.

In actuality, the upload refuses to proceed if the file selector dialog was cancelled rather than submitted before.