Handling non-image uploads when updating user photo from front-end form
Update:

As of the next (post 2.4.2702) Craft release, Craft will now return a validation error on the UserModel if a non-image is uploaded for the user's profile photo.



I consider this a bug in Craft and we've already got a ticket open to address it.

Until then, I can think of two options work work around it, either of them particularly pretty. 1. Hack core Craft files to do the detection. 2. Have some JavaScript on your upload form that detects the extension of the file they're trying to upload.