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.
In Visual Studio 2012: Right click on the project --> click Add --> click Web Form --> Copy the content of your original aspx file into the new WebForm aspx --> delete the original aspx file --> Rename the new one to anything you want. Now you should have a new aspx file with a code behind file that is ready for use