code: public function getMimeType(?IHttpBody $body): ?string
      {
          if ($body === null) {
              return null;
          }
  
          $parsedMimeTypeCacheKey = spl_object_hash($body);
  
          if (isset($this->parsedMimeTypeCache[$parsedMimeTypeCacheKey])) {
              return $this->parsedMimeTypeCache[$parsedMimeTypeCacheKey];
          }
  
          $fileInfo = new finfo(FILEINFO_MIME_TYPE);
  
          if (($mimeType = $fileInfo->buffer($body->readAsString())) === false) {
              throw new RuntimeException('Could not determine mime type of body');
          }
  
          // Cache this for next time
          $this->parsedMimeTypeCache[$parsedMimeTypeCacheKey] = $mimeType;
  
          return $mimeType;
      }
docs: Gets the MIME type of the body
  
  @param IHttpBody $body The body whose MIME type we want
  @return string|null The mime type if one is set, otherwise null
  @throws RuntimeException Thrown if the MIME type could not be determined