prevent users from uploading files other than images

This will show a message when the user tries to upload a file that is not an image.

![image](https://f.cloud.github.com/assets/362783/327556/b0ca7e94-9b79-11e2-919b-75ed9d7472b8.png)

**Note:** the test is currently *only* done client-side.
**Note 2:** this should be updated when we'll add [other file types support](http://meta.discourse.org/t/file-upload-support/2879/2).