Essence recognizes types in three ways (in order of precedence): by URL naming heuristics, by file naming heuristics, and by locating identifying data within a file using the UNIX file command.
To modify the type recognition step, edit lib/byname.cf to add file naming heuristics, or lib/byurl.cf to add URL naming heuristics, or lib/bycontent.cf to add by-content heuristics. The by-content heuristics match the output of the UNIX file command, so you may also need to edit the lib/magic file. See Appendix C.3 and C.4 for detailed examples on how to customize the type recognition step.