Fix call to form-data, to allow the setting of filenames in the attachment

If you check https://github.com/felixge/node-form-data/blob/master/lib/form_data.js#L151

form-data append expects the third parameter to be an `options` object which _may_ have a `filename` property .

This is a blocking bug for us, because one API we are consuming needs filenames to be not null.