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...