This sets the icon to display next to files with MIME-encoding for FancyIndexing. Icon is either a (%-escaped) relative URL to the icon, or of the format (alttext,url) where alttext is the text tag given for an icon for non-graphical browsers.
Mime-encoding is a wildcard expression matching required the content-encoding. Examples:
AddIconByEncoding /icons/compress.xbm
x-compress