Impossible to properly upload image if no proper mimetype guessers enabled

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