Remote avatar does not properly check if remote file is an image

The remote avatar should throw an error when an incorrect image link is supplied, i.e. something like this link to a 404 page on phpbb.com:
https://www.phpbb.com/avatar/55502f40dc8b7c769880b10874abc9d0.jpg

Instead, the avatar system thinks that a proper image has been supplied and updates the user's avatar data to point to this incorrect link. The "image" will not be loaded, however, this leaves us with a broken link to an image that should never happen in the first place.