Mime type is the "actual" type of the file. Many systems will check a file's extension vs it's mime type to verify that the image is what the extension claims it is. This can help prevent malicious code from being uploaded under the ruse that it's a valid image (though success may very).Edited By: Ulrike Rayne Schultheiss on Year 19 Day 97 10:11
If you're attempting to upload a custom or something similar to holocron, verify the file extensions. Older images uploaded through the custom system would have jpg as their extension regardless of their actual file type which would set off this error. If that's the case, the only other file type the system supports is png, so try switching the extension to png (gifs are automatically converted to jpg so they should never be a mime type combine can output) . This also still happens with avatars.