code: public void addFilePart(String name, String fileName, ContentProvider content, HttpFields fields) {
          addPart(new Part(name, fileName, "application/octet-stream", content, fields));
      }
docs: <p>Adds a file part with the given {@code name} as field name, the given
  {@code fileName} as file name, and the given {@code content} as part content.</p>
  
  @param name     the part name
  @param fileName the file name associated to this part
  @param content  the part content
  @param fields   the headers associated with this part