Support for content-type for the attachments.

The proposed changes add possibility to specify content types to the attached files. 
Something like this:

request.post(url)
        .query(reqPar)
        .attach(field, file, filename, contenttype)
        .end (res) ->

where contenttype is just a content type text like "image/jpeg" or something else. 
Sometimes you cannot trust on filename extensions.