HAProxy community

Simple Reverse Proxy (Proxy Pass) Short URL Style?

So I have HAProxy running on my firewall (opnsense). I am trying to use HAProxy on there to reverse proxy a few remote URLs and am struggling with how to do it. For example:

I have a web url that contains a text file (http://www.blah.com/myfile.txt), I want to have a url on HAProxy that reverse proxies that file (also obscuring the actual URL) like http://myfirewall.mydomain.com/blah.txt… should proxy the file from blah.com.

Does that make sense? I would do this in apache and nginx land with ProxyPass or similar.