What is the MultiMarkdown QuickLook Generator?
OS X offers support for QuickLook Generators --- small programs that are responsible for showing a preview of the contents of files. For example, if you select a file in the Finder and hit the space bar, you will see a preview of the file contents (assuming you have an appropriate QuickLook Generator installed.)
Fletcher T. Penney had a github project that provided a QL generator for MMD files( https://github.com/fletcher/MMD-QuickLook/). It worked, but had some limitations. This project still exists, but probably won't be updated much unless forked.
Rather than reinventing the wheel, he created a new QL generator that used code from MultiMarkdown Composer for improved performance and reliability, improved OPML support, proper handling of previewing local images, and some extra CSS to improve the visual design of the preview.
This new QL generator is free to download, but is not open
source. You can modify the embedded CSS file if you want to
customize it. Simply right click on the bundle to "Show package
contents" and then edit the embedded multimarkdown.css
file as desired.
This can be downloaded here