index-more: document format of contenttype-mapping.txt

The index-more plugin uses the conf/contenttype-mapping.txt file to build up the mimeMap hash table (in the readConfiguration() method).
The line splitting is performed around "\t", so it silently skip lines separated by simple spaces or more than one tab (see line 325).
Changing the single-char string "\t" with the regex "
s+" should do the magic.