SaveUrlParams question for Office Integrator

SaveUrlParams question for Office Integrator

I have the following setup for a CreateDocument call.  Other than the content, the other parameters seem to be coming in as null.  Is there something I am missing.  My current code:

var documentId = Guid.NewGuid().ToString();
...
SaveUrlParams = new Dictionary<string, object> 
{
{ "content", "$content" }, 
{ "fileId", documentId }
}
...
[HttpPost]
[Route("Save")]
public async Task Save(IFormFile content, string fileId)


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Additionally, I have modified it a bit and then that results in a 415 being returned. 

Alternatively, can an example be provided where a custom SaveUrl is used, and a custom parameter is passed pack in the SaveUrlParams, and what the Save API method looks like? The zoi-api-sdk Github examples are a bit lacking in this area.