Apache Reference: mod_autoindex, AddAltByEncoding

AddAltByEncoding

Alternative Text for File Icon (by MIME Encoding)
Syntax: AddAltByEncoding text mime-enc [mime-enc ...]
Example: AddAltByEncoding "GZIP" x-gzip
Since: Apache 1.0

This directive sets (one or more times) the alternative text to display for a file with the MIME encoding mime-enc, instead of an icon, for FancyIndexing. The mime-enc argument must be a valid MIME content encoding. The text argument is enclosed in double quotes. This alternative text is displayed if the client cannot show the image or has image loading disabled.