How to upload or download a file using the Bentley Web Services Gateway with eB Plugin - I'm getting 500 errors.

I am trying to create a new object in eB, with a file attachment, but I'm getting stuck - can anyone help me or should I raise a support request?

The steps I'm taking are:

GET https://<host>.bentley.com//ws/v2.8/Repositories 

GET https://<host>.bentley.com/ws/v2.8/Repositories/Bentley.eB--<host>-app.bentley.com,<name>-eb-db-prod-classic/MetaSchema/ECClassDef?$filter=SchemaHasClass-backward-ECSchemaDef.Name+in+['EB_ECPLUGIN_SCHEMA']

GET https://<host>.bentley.com/ws/v2.8/Repositories/Bentley.eB--<host>-app.bentley.com,-<name>-db-prod-classic/EB_ECPLUGIN_SCHEMA/Global_Document_Type/<id>/$file

This fails with the response:

{
    "errorId": null,
    "errorMessage": "The file (1218076) either does not exist or it is in a repository not accessible from this site or is not available at this time.",
    "errorDescription": null
}


Q1.) Why does this fail?  What am I doing wrong?

POST https://<host>.bentley.com/ws/v2.8/Repositories/Bentley.eB--<host>-app.bentley.com,<name>-eb-db-prod-classic/EB_ECPLUGIN_SCHEMA/Global_Document_Type



---------------------------acebdf13572468
Content-Disposition: form-data; name="My File.pdf"; fileName="My File.pdf"
Content-Type: application/pdf

<My File.pdf>

---------------------------acebdf13572468
Content-Disposition: form-data; name="instance"; fileName="blob"
Content-Type: application/json

{
  "instance": {
    "schemaName": "EB_ECPLUGIN_SCHEMA",
    "className": "Global_Document_Type",
    "properties": {
      "Code": "WSG-TEST-WITH-FILE",
      "Name": "WSG Test With File"
    }
  }
}
---------------------------acebdf13572468


This fails with the response:



{
   errorId: null,
   errorMessage: "Files cannot be added to Planned Documents.",
   errorDescription: null
}

Q2.) What does this error message mean? What am I doing wrong?

I then tried to create a new object, and upload a file in a second request.  The second request was

PUT https://<host>.bentley.com/ws/v2.8/Repositories/Bentley.eB--<host>-app.bentley.com,<name>-eb-db-prod-classic/EB_ECPLUGIN_SCHEMA/Global_Document_Type/<instanceId>/$file


This failed with a 500 server error.  POST does the same.

Q3.) Why is this causing a 500 error? What am I doing wrong?

I am unable to find any useful documentation on performing these seemingly routine steps, or any information on the errors I'm receiving, so any help you can offer would be really appreciated.