Multipart sets file name as the field/part name

[vincentjames501]

It looks like file name is conflated a bit in ->multipart-body. The file-name is optional yet it's always set to name. The field name and filename should be treated as separate entities. Does something like this seem reasonable?

(doseq [{:keys [name content part-name mime-type file-name]} parts]
      (.addFormDataPart builder (or part-name name) file-name (->request-body mime-type content)))

This allows us to set the name to something like "image" and file-name to something like "my-image.jpg" a bit more easily or simply omit the filename if we don't have one.