If you change a file name extension, the file may become unusable. Are you sure you want to change it?

You're renaming a file, and changing its file extension. This may make the file unusable. For example, ASP pages must have the .asp extension, and HTML pages must have the .htm extension, in order to be used by Microsoft^® Visual InterDev™.

It's generally safest to use the default extensions for file types.