Caching of web content on the RB750 is just silly, because you'll just damage the NAND memory of it, because it has limited amount of read/write cycles.
Unfortunately the HTTP Proxy on the MikroTik RouterOS is very limited, so you can't cache certain file/content types. The other issue is that most websites would provide you with a unique URL for downloading applications, so your proxy server will never cache it, unless you have a plugin for that specific site.
The same goes for many videos, like YouTube for instance, which is why you'll need a plugin for Squid to be able to cache it properly.
I'm using MikroTik RouterOS' HTTP proxy to cache general web content on my HP Microserver, just because I'm too lazy to setup Squid.
Update:
You can probably setup a Layer 7 rule to check for HTTP traffic's content type, but that would most likely max out your poor RB750's CPU.
The Web Proxy supports Web Proxy Cache rules, so you can try and add a rule to allow caching where the path ends with .exe or something like that, and deny everything else.
eg.
Code:
/ip proxy cache
add action=allow disabled=no dst-port="" path=*.exe
add action=deny disabled=no dst-port=""