Attach additional documents to an existing book entry that already has a main document. The result will contain the generated annexed document's id.

Use the multipart/form-data content type and add two arguments. The first one must be named file and contain the file's bytes, and the other one must be named body and contain the json-formatted information.

Supported document types :

• Pdf
• Xml
• CODA
• SODA
• Images (.jpg, .jpeg, .png)

The request will fail if no main document is attached to the book entry yet. In that case, use the endpoint 'book-entries.documents.new'.