That way, browsers will suggest to the users to upload an image file instead of any kind of file.
The behaviour is in-line with Element's, which specifies the same attribute when selecting an avatar.
Please note that it does not prevent users from uploading non-image files as avatars, as browsers interpret that attribute as a mere suggestion, which can be bypassed in the file select dialog.
Partially fixes#982.
9a34daa2bc