A POST endpoint has been hardcoded to stay in pure HTML. It most likely works, but you should
check for a working endpoint by clicking the following link.
You can then open devtools and change the "action" address to the one provided by the API.
https://firestorage.jp/jsup.cgi
Once the upload is finished, you will receive a URL-encoded response.
Paste it into urldecoder.org and decode.
The download link is enclosed in quotes and begins with http://firestorage.jp/download/.
You can share this link.