docs: def encode_multipart_formdata(fields, boundary=None):
      """
      Encode a dictionary of ``fields`` using the multipart/form-data mime format.
  
      :param fields:
          Dictionary of fields or list of (key, value) field tuples.  The key is
          treated as the field name, and the value as the body of the form-data
          bytes. If the value is a tuple of two elements, then the first element
          is treated as the filename of the form-data section.
  
          Field names and filenames must be unicode.
  
      :param boundary:
          If not specified, then a random boundary will be generated using
          :func:`mimetools.choose_boundary`.
      """
      # copy requests imports in here:
      from io import BytesIO
      from requests.packages.urllib3.filepost import (
          choose_boundary, six, writer, b, get_content_type
      )
      body = BytesIO()
      if boundary is None:
          boundary = choose_boundary()
  
      for fieldname, value in iter_fields(fields):
          body.write(b('--%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(b('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(b'Content-Type: text/plain\r\n\r\n')
  
          if isinstance(data, int):
              data = str(data)  # Backwards compatibility
          if isinstance(data, six.text_type):
              writer(body).write(data)
          else:
              body.write(data)
  
          body.write(b'\r\n')
  
      body.write(b('--%s--\r\n' % (boundary)))
  
      content_type = b('multipart/form-data; boundary=%s' % boundary)
  
      return body.getvalue(), content_type