This can be helpful for example when a file extension type opens with the wrong program by default, has the wrong icon, or is missing or corrupted in the registry. Downloading the .reg file for the selected file extension in the tutorial below, will restore that file extension back to default for all of these items above.
Since there are so many default file extension types in Windows 7, it may take a bit to have a complete list of them below. If the file extension type you need to restore is not listed below yet, then please post a request for it. I'll be happy to add it to the list.