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).
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.
Edited By: Ulrike Rayne Schultheiss on Year 19 Day 97 10:11 ____________
I've found this most commonly happens if you right click and save an image from someplace (like SWC) and go to upload it, and to fix it I just open it in photoshop and resave it as whatever filetype. Really not sure why saving straight from SWC seems to cause this so frequently, but I've run into it a ton when copying GNS articles over.
Syn, a lot of our images have the wrong extension. The custom system itself used to love using .jpg for every image regardless of its actual file type. The same goes for a lot of the official images. Many official images have .jpg because the code didn't always support files that ended in other extensions. It's a bit of a guessing game on what the image type actually is here, unfortunately.