Fix .doc export with LibreOffice (soffice) (#3338)
Fix .doc export with LibreOffice (soffice) (#3338)

When using LibreOffice to convert pads to doc, we got `Error: no export filter for /tmp/xxxx.doc` (tested with LO 5 and 6). Maybe it's a regression from LO. Anyway, converting HTML to odt, then to doc works.

Thx to lpagliari for her review!
storytouch/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,joequant/etherpad-lite,aptivate/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,lpagliari/etherpad-lite,lpagliari/etherpad-lite,storytouch/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,University-of-Potsdam-MM/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,lpagliari/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,lpagliari/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite