code: def encode_multipart_formdata(fields, boundary=None):
      
      body = StringIO()
      if boundary is None:
          boundary = mimetools.choose_boundary()
  
      for fieldname, value in fields.iteritems():
          body.write('--%s\r\n' % (boundary))
  
          if isinstance(value, tuple):
              filename, data = value
              writer(body).write('Content-Disposition: form-data; name="%s"; '
                                 'filename="%s"\r\n' % (fieldname, filename))
              body.write('Content-Type: %s\r\n\r\n' %
                         (get_content_type(filename)))
          else:
              data = value
              writer(body).write('Content-Disposition: form-data; name="%s"\r\n'
                                 % (fieldname))
              body.write('Content-Type: text/plain\r\n\r\n')
  
          if isinstance(data, int):
              data = str(data)  # Backwards compatibility
  
          if isinstance(data, unicode):
              writer(body).write(data)
          else:
              body.write(data)
  
          body.write('\r\n')
  
      body.write('--%s--\r\n' % (boundary))
  
      content_type = 'multipart/form-data; boundary=%s' % boundary
  
      return body.getvalue(), content_type
docs: Encode a dictionary of ``fields`` using the multipart/form-data mime format.
  
      :param fields:
          Dictionary of fields. The key is treated as the field name, and the
          value as the body of the form-data. If the value is a tuple of two
          elements, then the first element is treated as the filename of the
          form-data section.
  
      :param boundary:
          If not specified, then a random boundary will be generated using
          :func:`mimetools.choose_boundary`.