How to attach docs and import issue at the same time using GitHub API?

I need to import all bug reports and attachments from my internal system to GitHub Issues.

What GitHub API parameter should I use to attach a document in my curl command?

curl -X "POST" \
     -H "Accept: application/vnd.github+json" \
               -H "Accept: application/vnd.github.VERSION.html" \
     -H "Authorization: token <token>" \
               https://api.github.com/repos/BT23/demo-repo/issues \
               -d '{
               "title":"Create an issue in HTML format",
               "body":"<H1>Testing HTML tags</H1><P>Does it work?</P>",
               "assignee": "BT23",
               "milestone": 1,
               "labels":["bug", "functional"]
               }'

Thanks