metadata

If you mean you want to set custom metadata for files that you could read via the API without downloading the file content, then no, this isn't currently possible, but I'll be sure to pass this along as a feature request. 

If you mean you want to set metadata specific to certain file types though, that would be stored in the file itself, you can download the file, edit it as desired, and upload the new version.

Yes, my question is about  to set custom metadata for files that could be read via the API. I hope you can elevate this to a feature request. 

Thanks!

The Dropbox API now offers the ability to add arbitrary key/value data to Dropbox files via "file properties":

https://www.dropbox.com/developers/documentation/http/documentation#file_properties

If you're using an official SDK, there will also be corresponding methods for these endpoints.