So I attempted to upload a new banner I'd received a few days ago, and it gave the "invalid file type" message. So I converted it to a JPG, but it still won't upload and keeps telling me the file type is invalid.

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This is what pops up

I have had the same problem... its not uploading ;(

So I tried a couple things to see when the error message you're getting showed up.

Are you sure its .jpg an not .jpeg? I saved a banner that definitely uploads as a .jpeg and got the same message.

I used an image converter that was strictly .jpg, and it's worked before loads of times.