The first question people ask at this point is "What properties can I write to what filetypes?".  They usually expect a concise answer at this point, too.

The short answer is that "It depends on the filetype."  Not all filetypes support writing properties.

The real answer is that we haven't compiled a table with this information yet.   Do I hear you asking why?  We don't want to leave you in the dark -- it's a lot more mundane than that.  We simply let each filetype owner make these decisions independently and have not collated the results yet.  I hope to blog about individual filetype support at some point, but for now you will just have perform a few guided experiments.  There are also a number of algorithms and data sources that may address your needs.  Stay tuned...

-Ben Karas