Markdown API from file

How use markdown API from file?

I want to use markdown API for render markdown to html from file
(In file only markdown content (not json)).

I see possibility use only text in query parameter or body.

GitLab Version: 12.8.5

I read gitlab api documentation (help): https://docs.gitlab.com/ee/api/markdown.html

text1337×680 47.3 KB

file1366×943 155 KB

Please help me.