ie: hidden extension you mean Only A Hobo. Well any file that has a hidden extension is only hidden because they don't want you to know about it. ie: wma/wmv files are potentially dangerous!
Stick to the normal video downld rules/routine: Movie download issues
Size is the important factor backed up with a Bitzi search if it exists on their database: How to use Bitzi Web Lookup ... Perhaps I'm over-simplifying it a bit ... but I'm sure you know what I mean.