To specify a file name for the index:
[Index] ; IDXFile = base name to use for output file, with suffix IDX IDXFile = mapnameIDX ; IDXSuffix = suffix for the base file name, default IDX IDXSuffix = IDX ; IndexFileSuffix = suffix to add to map name for index file ; if no IDXFile name (deprecated) IndexFileSuffix = IDX.htm
If you specify a value for IDXFile, IDXSuffix is ignored. See §14.2 Naming generated HTML list and index files.
For IndexFileSuffix, you must include the file extension as well as a suffix to add to the base map name. The default suffix is IDX.htm. Use of IndexFileSuffix is deprecated. If you provide a value for IDXFile, DITA2Go ignores IndexFileSuffix.