Yes you may be able to using regular expressions if that proves to be too much of a challenge you can think about investing in the CSC-SSM which is an expansion module for the ASA that filters http (among other things) and offers some flexibility in terms of the file types that can be downloaded.
If you want a more robust solution I would recommend WebSense or IronPort. Hope that helps.
You are limited to what you will be able to do with the ASA 5520 that you have. For a more robust solution you will need to think about investing in either CSC-SSM, WebSense or IronPort. Please check link below:
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...