how to add custom thumbnailers to thunar?

[Auguste Pop]

hi,

according to this page
(http://thunar.xfce.org/documentation/C/customizing-thunar.html),
which seems outdated, custom thumbnailers can be added by a desktop
file. unfortunately, the command it uses to make the thumbnailer
recognized by thunar is no longer available. after some google search,
i found out that thunar seems to be using tumbler to handler all the
thumbnail generation now. but i can not find a clear documentation on
how to add custom thumbnailers to tumbler. where can i find relevant
information on this?

thank you for your kind attention.

best regards,