Message on unrecognized image is wrong

When uploading an image that is not supported the error given implies a mismatch on mimetype.

The file 'RAW_PANASONIC_LUMIX_FZ18.RAW' content does not match with the given MIME type: image/x-raw-panasonic

However what is really going is that the image is not recognized as a known image type. So basically we do not support that image type and the check done has nothing to do with mimetype.