docs: async def convert_endpoint(url_string, ts, is_just_checking):
      '''
      Main logic for HTTP endpoint.
      '''
      response = singletons.server.response
  
      # Prep ForeignResource and ensure does not validate security settings
      singletons.settings
      foreign_res = ForeignResource(url_string)
  
      target_ts = TypeString(ts)
      target_resource = TypedResource(url_string, target_ts)
  
      # Send back cache if it exists
      if target_resource.cache_exists():
          if is_just_checking:
              return _just_checking_response(True, target_resource)
          return await response.file(target_resource.cache_path, headers={
              'Content-Type': target_ts.mimetype,
          })
  
      # Check if already downloaded. If not, queue up download.
      if not foreign_res.cache_exists():
          singletons.workers.enqueue_download(foreign_res)
  
      # Queue up a single function that will in turn queue up conversion
      # process
      singletons.workers.enqueue_sync(
          enqueue_conversion_path,
          url_string,
          str(target_ts),
          singletons.workers.enqueue_convert
      )
  
      if is_just_checking:
          return _just_checking_response(False, target_resource)
  
      # Respond with placeholder
      return singletons.placeholders.stream_response(target_ts, response)