Apple - Customize Quick Look to allow me to view .yml files

I've had great luck with the free QLStephen plugin for Quick Look. It displays lots of other plaintext formats.
Probably worth a shot to see if it will work with YAML files.

You can get it here.

You can use the QLStephen plugin as mentioned but you will need to edit it as described in the project's issue tracker: https://github.com/whomwah/qlstephen/issues/23#issuecomment-21769063

Basically:

1. Open the ~/Library/QuickLook/QLStephen.qlgenerator/Contents/Info.plist file in a text editor
2. Add the UTI public.yaml to the key LSItemContentTypes
3. Run qlmanage -r to reload the service

I use Ciarán Walsh’s TextMate in QuickLook plugin.

This is a TextMate plug-in and a QuickLook generator which renders QuickLook previews (for certain filetypes) using the TextMate syntax highlighter. Currently it’s configured to highlight Perl, PHP, Ruby, Shell, C headers and Obj-C(++). Note: this effectively sets up TextMate as a server for the syntax highlighting, so it won’t work unless TM is running (QuickLook will just fall back to the standard preview if it’s not).

Here is a convenience link to download the plugin and you can read more about it on Ciarán's web page.

This is what it looks like when I QuickLook a .yml file: