Image type validation

Validate image type before uploading. Valid types are png, jpe?g and gif.

Before this change, we tried to upload any type of file. The result was:

IE users: Image downloading hands to "Processing..." state
Others: Image starts to download, but fails with error message "Image download failed"

Now all users get better error message "The image file must be either in GIF, PNG or JPG format".

The validation part was already implemented, we just didn't react on validation failure.