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May be a rare encounter for most people, but Image Manager in 2.0.10 doesn't seem to like image files when the file extension is in caps, e.g:
myfile.JPG
It throws the error message:
Unknown filetype rejected
Mime type not allowed: image/jpeg
I would think it is only likely be a problem if you use an old collection of images, as Photoshop 4 saved files with the extension in caps by default.
Changing to lower case solves it.