Adding 64x64 images for file types.

These images will be displayed based on the filetype whereever files
are displayed.
Removing the unused variable fileUrl from UploadHelper.php file
Establishing the relationship between comments and uploads.
Adding code to get the image URL and to get the icon file of the upload

Also a function is written to check whether the file is image or not.
We will be using this to display the lightbox overlay. We will only be
returning true if image is of jpg, png or gif.
Added code to display the comment uploads.