After double-clicking on the unknown file icon, the system should open it in the default software that supports it. If this does not happen, download and install the TextMate software and then manually associate the file with it.
If there is no TextMate or similar software in the system that supports files with the MD extension, you must first download and install it. Below you will find a list of the most-used applications that work with MD. After going to the subpage of the program you will find a link to the developer's website, where you can safely download the software installer.