fix target format for external format-filter-server

According to https://github.com/owncloud/format-filter-server/blob/cc435eac37fb6e67f73fb47d404281c59aa49bd7/code/conversion.js#L17 the target_format should be "odt".

The format-filter-server returns an empty response otherwise.
Merge pull request #376 from aaukt/external-ffs

fix target format for external format-filter-server