mimeconf specifies how the
different MIME types are handled for indexing, and which icons
are displayed in the recoll result lists.
Changing the parameters in the [index] section is probably not a good idea except if you are a Recoll developer.
The [icons] section allows you to change the icons which
are displayed by recoll in the result
lists (the values are the basenames of the png images inside
the iconsdir directory (specified in
recoll.conf).
