I'm creating this issue as a MDLSITE, as I can't think of a way to reproduce it but its probably a MDL bug.

Adding a plugin to the plugins database and adding a screenshot, I dragged and dropped my file onto the filemanager, and got an error message 'Image (PNG) type cannot be accepted'). However uploading the file manually using the upload repo it worked fine.

This was the the same with both the screenshot and icon field