(Sorry, I'm not super familiar with MHTML files, so I'm going to assume these are just static files that a browser can interpret.)
There is nothing in particular in Rails that provides the behavior that you're referring to. Rails is just a framework to help serve dynamic responses, and the functionality you want is in relation to the filesystem, which Rails mostly abstracts away.
There's a couple options I can think of.
Roll your own
Ruby has file and directory utilities which you could use to read in the contents of a directory in public and render the kind of response you'd like. I would start here:
http://www.ruby-doc.org/core-1.9.3/Dir.html
And play around with ruby's directory globbing to get a file listing.
Use Rack
Rails is built ontop of Rack, a standard webserver interface that most ruby application server implement now. Rack provides some libraries that make serving static assets like a traditional web server a little easier.
Here's a couple of resources to check out:
http://quickleft.com/blog/rack-130-serving-static-files http://edgeguides.rubyonrails.org/rails_on_rack.html