It's currently impossible to upload an image if the mimetype guesser doesn't have any proper mimetype guessers enabled. Even if the browser sends the image/png mimetype using the upload form, the mimetype guesser will overwrite it with application/octet-stream as it's not able to properly guess a mimetype. This will prevent people from uploading perfectly valid images on i.e. Windows and should be fixed by undoing this commit:
https://github.com/phpbb/phpbb/commit/4eb7485b397733c8765f8bd37a04b7a034b29792