you have two proxy server, both of them are MT OS or other operating OS like linux? If you have 2 MT OS then why u want to separate http and other files like exe. mgp. zip etc? you can use 1 MT for your normal NAT and other firewall setting, and with 2nd MT OS you can make that only as a web proxy, I'm using this kinda setting in my network and having great performance with my web proxy. I have told about this in the "Mikrotik + Web Proxy Queueing is Impossible?" topic. but I think none have any faith on my solution huh huh huh.
N.B: You can also user your proxy server with linux(squid), it'll also work just fine.