muCommander is a cross-platform file manager. It has it’s own file editor/viewer but if you want to use your favourite one, there is a chance. I prefer to use macvim on Mac and as preview action I like what Finder does, so see what you need to add to ~/Library/Preferences/muCommander/preferences.xml file.

<editor>
    <use_custom>true</use_custom>
    <custom_command>open -a macvim $f</custom_command>
</editor>
<viewer>
    <use_custom>true</use_custom>
    <custom_command>qlmanage -p $f</custom_command>
</viewer>

Tom. Developer and Musician. The man behind Virtual Analogy and its products.