code: public function getMimeType($request): ?string
      {
          if (!$request instanceof IHttpRequestMessage && !$request instanceof MultipartBodyPart) {
              throw new InvalidArgumentException(
                  'Request must be of type ' . IHttpRequestMessage::class . ' or ' . MultipartBodyPart::class
              );
          }
  
          return $this->bodyParser->getMimeType($request->getBody());
      }
docs: Gets the MIME type of the body
  
  @param IHttpRequestMessage|MultipartBodyPart $request The request or multipart body part to parse
  @return string|null The mime type if one is set, otherwise null
  @throws InvalidArgumentException Thrown if the request is neither a request nor a multipart body part
  @throws RuntimeException Thrown if the MIME type could not be determined