Seems qbittorrent plugin will crash in part because when adding a torrent multipart files is used. But no files are actually given to the web api. @dsemi please update your plugin to post data= not files= as files require that you send the downloaded files from the download output. And from looking at the code. on_task_download is not needed as qbittorrent will retrieve the files itself similar to deluge.

https://github.com/Flexget/Flexget/blob/develop/flexget/plugins/output/qbittorrent.py#L79
that file is never given to qbittorrent. No need for it to download a temp file if it's never sent.
https://github.com/Flexget/Flexget/blob/develop/flexget/plugins/output/qbittorrent.py#L65
That is not needed for a json post to the webui and the entire line can be removed. Unless you plan on sending the actual temp file you downloaded in the on_task_download. At which point files can be used.
https://github.com/Flexget/Flexget/blob/develop/flexget/plugins/output/qbittorrent.py#L66
that should be

self.session.post(self.url + '/command/download', data=data)