Set `accept` attribute to `image/*` in ImageUpload (#989)

Set `accept` attribute to `image/*` in ImageUpload (#989)

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.