How to add custom extension for QuickLook? Does anybody know of a method of previewing .WebM files in QuickLook? 
 I've searched on google but didn't find anything helpful.
You could try adding it via the .plist file.

This guide will explain how to do it.

To summarize the steps:


Go to the applications folder
Locate TextEdit.app
Show Package Contents
Edit the Info.plist file