QuickLook set app to display preview
qlmanage

Use the command line tool qlmanage to investigate your QuickLook set up, including the default generator for each file format.


  qlmanage -- Quick Look Server debug and management tool


Apple's QuickLook developer documentation provides a good overview of how QuickLook works and how to test specific plugins.

Editing the iA Writer QuickLook Plugin

You may want to edit the iA Writer QuickLook plugin to disassociate it from your .h and .m files. See QuickLook file associations for more information about achieving this.