Q:

How do I find (or create) a File Template override in Panels 3? 

So I'm using Panels 3 and Drupal 7 and it makes it damn easy to override the default templates for a taxonomy term template, a node template, even the front page template.   However, I don't see anything on the default panel page admin list that allows me to override the default FILE template.   When I try to Add a New Page and use the file/%file path, the system forbids it, as this URL structure already exists.   How do I harness the power of Panels for other page templates that the Panels module does not offer an override out-of-the-box? 

A:

As Adam Friedman points out, there is Page manager existing pages, which can do part of what "real" Page manager support can do, but it will not have the same capabilities as say the node/%node template has. 
There are two issues that work on "full" support for this.  First, and Second.