I just inherited a web site that was created by a designer. The site was originally created with all *.html files. The designer renamed all the *.html files to *.aspx files.
After you add the new .cs file, you may want to see the file look like a codebehind file (indented, icon, etc). To do so:
For a file Profile.aspx.cs, the xml should look something like this:
Profile.aspx
ASPXCodeBehind