finally we have an API documentation… 
can you explain, what you are trying to do? creating a media item?
as a start, you could first try the LIST option, which should be simple to implement
GET /wp/v2/media
like this you can test, if your Authorization headers are correct…
and if you got that running, then continue with your other task…
and concerning Authentication, I would follow this
and use Basic Http Authentication…
see also here App Inventor Tutorials and Examples: File Download | Pura Vida Apps
Taifun