MultipartFile.fromString(String field, String value, { String filename, MediaType contentType })

Creates a new MultipartFile from a string.

The encoding to use when translating value into bytes is taken from contentType if it has a charset set. Otherwise, it defaults to UTF-8. contentType currently defaults to text/plain; charset=utf-8, but in the future may be inferred from filename.

Source 

factory MultipartFile.fromString(String field, String value,
    {String filename, MediaType contentType}) {
  contentType = contentType == null ? new MediaType("text", "plain")
                                    : contentType;
  Encoding encoding = encodingForCharset(contentType.parameters['charset'], UTF8);
  contentType = contentType.change(parameters: <String, String>{'charset': encoding.name});

  return new MultipartFile.fromBytes(field, encoding.encode(value),
      filename: filename,
      contentType: contentType);
}