code: public function shouldPostFieldsBeSigned(Request $request) {
          $returnValue = false;
  
          if ($request->hasHeader('Content-Type')) {
              $contentType = $request->getHeader('Content-Type');
              //TODO - not safe
              if ($contentType === 'application/x-www-form-urlencoded' ||
                  $contentType === 'multipart/form-data') {
                  $returnValue = true;
              }
          }
  
          // Don't sign POST fields if the request uses POST fields and no files
          $body = $request->getBody();
          
          if ($body instanceof \Amp\Artax\FormBody) {
              if ($body == 0) {
                  $returnValue = false;
              }
          }
  
          return $returnValue;
      }
docs: Decide whether the post fields should be added to the Oauth BaseString.
  Flickr incorrectly add the post fields when the content type is 'multipart/form-data'. They should only be added when the content type is 'application/x-www-form-urlencoded'
  
  @param $request
  @return bool Whether the post fields should be signed or not