Myself and I'm sure many others have struggled with how to add custom file types to the preview pane for windows 7. While the process for adding file types for vista was already worked out
Add File Types to Vista's Preview Pane
no workaround was available for windows 7 that I was able to find so I thought I would show you how to do that.
First use the manual steps or the automated tool(PreviewConfig) from the how-to geek article above on the file extension of your choice.
For this article I'm going to add .nfo files
open up your registry editor by typing regedit in the start menu's run dialog and hit enter
navigate your registry key tree to computer\HKEY_CLASSES_ROOT\.nfo You should see a string value named default, with a type of REG_SZ, and containing the data MSInfoFile.
now follow either the manual process or used the automated tool from the How-To Geek article above to add your custom file types to the preview pane. You only have to create two new string values and fill in their data in addition to that to get preview working for custom file types in Windows 7. Since I'm trying to add a plain text file type to the preview pane I can take a file type that I know previews correctly say .txt files in my case, and make identical copies of the two string values which are missing from my computer\HKEY_CLASSES_ROOT\.nfo key. Those values being named "Content Type" and "Perceived Type" respectively. The Content Type string value should have the data of "text/plain", and the PerceivedType string value should have the data of "text"
That's it if you followed the directions correctly in both the guide for doing it in vista either doing it either manually or with PreviewConfig, and the directions here you should now have the ability to preview .nfo files. This doesn't always work for instance I couldn't get .mkv or matroska video files to preview correctly or even open using the preview provided shortcut to open in windows media player even though they would if i just chose open with windows media player by right clicking a .mkv file