How to save content with its related relation content type

[Alex Mika]

Since i did not find any posibilities to add a filed which gives me advantage to create a content with multipart files (more then one binary file), i created relationship between a content type which includes meta data, and a relationship fileds to another content type which include binary file upload. See the screenshot. 

My question is, how can i now use rest api to save this kind of content?