I have had this error come up once or twice. In my case, it was that the image host sat the JPG images behind a HTML page with a JS challenge to prevent hotlinking and scraping of images. When phpBB went to check the image dimensions, it saw a HTML file, not a JPG.
In essence, It's the same error as described above, because when phpBB checks the file, it is an unexpected MIME type / extension.
I think what is confusing here is that the error refers to not being able to determine image dimensions, rather than incorrect MIME type. Perhaps the error message should be changed to indicate the image has the wrong extension or MIME type eg
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Error: unexpected image extension or MIME type. Make sure the file type and extension for the file you entered is correct.