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I am running into an issue where OS X doesn't add extensions to .png files. Users are attempting to upload these files and get a file type not allowed error when they shouldn't. I would like to see the upload verify first by extension, then by MIME type (adding the appropriate extension to the file name if it passes verification).
I am assuming a similar issue could be found with any image file as well as text and video (if not with any file) and it seems to me to be a good idea to make the module accept images from users who don't know how to add an extension themselves.