Error message / Warning for invalid file uploading

when you try and upload an invalid file, an error message or warning should be displayed.
At the moment it does not upload the invalid file as expected but it doesn't let you know that it didn't.

Start by locating the file-upload entry point in the JavaScript frontend and trace how invalid files are currently rejected. Confirm how valid and invalid files are distinguished, then make sure invalid uploads produce a visible error or warning while valid files continue to upload.

Written by the indexing model from the issue text.