Current versions of WebKit and Safari 5 prevent the opening of HTML files with arbitrary extensions (i.e. .mako). This prohibits the viewing of templating files that are HTML-based. Also, affects embedded WebKit apps.
Repro: 1.) Create a sample HTML file named test.mako. 2.) Attempt to open it via File->Open File. You will be prohibited as the .mako file will be greyed out in the file dialog.
The dialog presented by File -> Open is part of the Safari application, not of the WebKit rendering engine. Bugs related to the Safari application should be filed at <http://bugreport.apple.com/>.