I don't have enough reputation to add a comment, but, the solution to your problem is the correct answer from this thread: Windows 7 - Preview other file types as text in Preview Pane
Edit: Actually, according to this guy in that same thread, using PreviewConfig should be enough.
Edit 2: In order to preview custom or non-standard file-types with actual rendering (eg. color for code, rendering markdown, or images), it seems you need to have a Preview Handler. This thread on another forum adds some useful information to the discussion of developing such a preview handler, particularly this tutorial written in C#. I tried searching for a pre-written .md preview handler, but have come back empty handed. If you know C# I highly suggest looking further into this.