Rack::OptionalHtml middleware handles extensionless pages on a Middleman server
If there is no match for the url, it will try the same url with some different versions of a .html suffix.
The effect is similar to using the Directory Index extension, but is less intrusive in that it will not introduce extra directories, rename the actual page to index.html, etc. It also makes it possible to call images by filename only.
On the down side, it will only effect the Middleman development server, so the production server needs to be configured to behave in the same way.
The gem can be found here: https://github.com/tommysundstrom/middleman-rack-optional-html