This file is used for mapping file types to particular icons. It is located at Program Files\Common Files\Microsoft Shared\web server extensions\50\Templates\1033\xml.
Generally, the CAML elements that you want to use in customizing icons for file types include:
To add a .wmf (Windows Metafile Format) file name extension, add the
following line to the ByExtension section in this file—
Key="wmf" Value="icwmf.gif"/>—and then add the file icwmf.gif to the
Program Files\Common Files\Microsoft Shared\web server
extensions\50\Templates\1033\Images directory. This allows you to add .wmf
files to your team Web sites and causes the icwmf.gif icon to show up beside
them, indicating what kind of files they are.
For a more complete example, see Example 1 - Adding a new field type for Flash movies.