Perhaps it's there already and I'm missing it but as per my note:http://urdland.com/forum/index.php?topic=276.msg1849#msg1849
Working like this:
New download option to "Check Content"
Soon after the DL starts run ''unrar -l' against the files in the tmp folder
If the content does not contain the expected extension inform or give the option to cancel the download.
It could also be a system option:
"Always abort downloads of unexpected file type"