Do you have mimeinfo.cache file? It should be located somewhere under /usr/share/ directory (for me it's /usr/share/applications/mimeinfo.cache) If you have it look through it for stings like
Code:
text/rtf=libreoffice-writer.desktop;
Try to add this string if such a string missed
Another one way to open document with the LibreOffice is: right click on file -> Properties -> File Type Options. There you can associate any program to particular file type.