The initial version of LibreOffice used the OpenOffice.org mimetype icons that were in place from OOo version 3.0 to 3.2.
In LibreOffice 3.3.1 and later our own iconset is used. It has been developed in a short timeframe by a collaborative action among the LibreOffice Design Team.
This is how they look like:
As part of our branding overhaul (we want to replace the initial branding by a community based branding) the icons will be reconsidered as well. Possible ideas for new icons are presented on the Proposals wiki page.