Insert a financial operation. The result will contain the generated book entry's id.

There are two ways to insert it, either with or without an attached document.

• When no document is attached, use the application/json content type as usual.
• For joining a document to the entry, 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.