iRule to Allow A Specific URL to Access A Filetype

This needs clarification - how can a URL access a file type? users access URLs which might end in a file type. I am guessing you probably mean an IP address not a URL?

My understanding of your requirements is the following:

• You have an ASM policy assigned to multiple Virtual Servers (different websites)
• This ASM policy is only allowing access to standard web files (i.e. .html .jpg .css .js) and is not not access to .log files
• As an exception you want to allow one client identified by something (you say URL, but is it a referrer URL, or an IP address?) to bypass the ASM and request a .log file from one of the virtual servers without being blocked by ASM.
• All other clients/users must still be blocked whenever they try to access a .log file type
• You want to use an iRule for this functionality (which is fine you can use ASM::unblock command for example)

Please note that this can be solved with a local traffic policy on the virtual server to save you from the complexity of writing and maintaining an iRule.