Future<MultipartFile> fromPath(String field, String filePath, { String filename, MediaType contentType })

Creates a new MultipartFile from a path to a file on disk.

filename defaults to the basename of filePath. contentType currently defaults to application/octet-stream, but in the future may be inferred from filename.

This can only be used in an environment that supports "dart:io".

Source 

static Future<MultipartFile> fromPath(String field, String filePath,
    {String filename, MediaType contentType}) async {
  io.assertSupported("MultipartFile.fromPath");
  if (filename == null) filename = path.basename(filePath);
  dynamic file = io.newFile(filePath);
  int length = await file.length();
  ByteStream stream = new ByteStream(DelegatingStream.typed(file.openRead()));
  return new MultipartFile(field, stream, length,
      filename: filename,
      contentType: contentType);
}