Unable to update file using staging

Hi,

I am trying to update a file in a document table using stage_edit with Postman. Everything looks to work fine. All the right status codes, but nothing happens. The record remains unchanged. The steps I take to perform the update are:

1. create a stage which returns a 201 Created and a stage_id: POST .../document(document_id=50)/stage_edit
2. then using a PATCH .../staged_document(<STAGE_ID>) I assign the new filename, in our case "storage_location": "xyz.pdf" using a JSON payload which returns a 200 OK and "resource_changed": true in the result body
3. then I upload the physical file (a local PDF) using POST .../staged_document(<STAGE_ID>)/upload_storage_location which results in a 204 No Content
4. and to finish up I commit the stage using POST ../staged_document(<STAGE_ID>)/commit which results in a 201 Created and the correct Location value in the headers

The initial creation of the document record follows a similar path using stage_add and creates the document-record with attached file in the storage_location correctly.

Any idea what I might be missing?