code: protected function buildMultipart($mimeBoundary, $filename)
      {
          $binary = $this->getBinaryFile($filename);
  
          $details = pathinfo($filename);
  
          $type = $this->supportedMimes($details['extension']);
  
          $data = '--' . $mimeBoundary . static::EOL;
          $data .= 'Content-Disposition: form-data; name="media"; filename="' . $details['basename'] . '"' . static::EOL;
          $data .= 'Content-Type: application/octet-stream' . static::EOL . static::EOL;
          $data .= $binary . static::EOL;
          $data .= '--' . $mimeBoundary . '--' . static::EOL . static::EOL;
  
          unset($mimeBoundary, $filename, $binary, $details, $type);
  
          return $data;
      }
docs: Build a multipart message
  
  @param string $mimeBoundary MIME boundary ID
  @param string $filename File location
  @return string  Multipart message