Image File Formats

As an app developer, I want to be able to assume that image field types only accept images so that I can avoid trying to guess all the ways my users might do it wrong.

Right now, if a user tries to upload an excel file, they get an error right at the choose file prompt that lets them know it wasn’t uploaded an a pink error that says:
Command failed: gm identify: Unable to open file (/tmp/gm88xXuH) [Invalid argument]. gm identify: Request did not return an image.

But, without custom validation, users can upload PDF files, even though PDF files are not images. I did not go through all the various possible file times to see what could and couldn’t be uploaded as an image. I would expect that the developers would have limited the file types to those that can be displayed by html as an image, though.