EDIT
Ok am back - Its looking good so far - done a few test posts with 1 preview pic and 1 small test rar
The rar wont download if got no credit which is fine but the pic loads fine

will do a few more tests to see if its 100% but so far its looking good.
EDIT (again)
Ok can confirm that the code that Death posted does show image files while making people pay a credit to download the attached rar.
add the code in attachment.php