DOCICON.XML
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:
MapToIcon — This element allows you to change the icons for the specified file name extension and ProgID.
ByExtension — This element can be used to map a file type (like .xls) to the appropriate icon (for example, to a Microsoft Excel file type icon).
Example
To add a .wmf (Windows Metafile Format) file name extension, add the following line to the ByExtension section in this file—<Mapping 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.