If you try to edit a markup document with a non-standard file extension such as .tpl or .mgi you'll find that GoLive might open the page showing only the source code with no Layout editor. The good news is that you can teach GoLive how to handle these non-standard file extensions in just a few easy steps.

Start be selecting Edit>Web Settings... where we'll create a new File Mappings entry for the unrecognized file.

Select the File Mappings tab on the far right of the Web Settings dialog and reveal the list of entries in the text section as seen below.

In most cases you'll want GoLive to treat markup documents with non-standard file extensions as if they were HTML files. If that's the case, scroll down to select the entry for html and select Edit>Duplicate (Command/Control-D) to create a starting point for your new entry.

With the duplicate still selected, just enter the new file extension in the Suffix field of the Inspector palette. When you're done addding File Mappings close the Web Settings window and the next time you open a document with that file extension GoLive should open the file as if it's a plain HTML file. If there's a file extension you think should be added to GoLive's default settings in a future version please send an email to golivewishlist@adobe.com with the file extension and a brief description of the file type.

Bonus Tip: You might notice your new entry is not automatically added to the correct category. The next time you open the Web Settings window GoLive will refresh the view and categorize the entries as needed.