POST /documents/{id}/files.html
When looking to upload a file to the document a POST call must be made to documents{id}/files.html. A file object is expected in the request body. Once it reaches the server, this file will be appended to any other existing files in the current document or it will be set as the first if there are no other files.
HTTP POST /documents/{id}/files.html
A successful response message will be as follows:

The success message will return the uploaded file information as well as the standard success status of the API call.