Since Middleman only generates static HTML files, it is up to the web server to determine what urls are used to access each file. With Apache, for example, about.html can be accessed at
Multiviews is enabled. Therefore, depending on how you serve your site, you will need to configure the web server in question.
On the other hand, consider setting the canonical url tag. It will tell search engines the correct url of a page, when it is accessed with an alias, among other benefits.
Hope this helps.