FrameMaker Server: Passing JSON data via Windows cURL?

Colleagues,

I'm using the FrameMaker Server 2020 REST API on Windows 10, invoking API calls via cURL for rapid prototyping.

All works as expected, _except_ when I try to POST a JSON file with parameters for the operation (example operation: creating a publishing task).

My cURL command line for creating a task looks something like this:

C:\Users\Administrator>curl -v -H "Content-Type: application/json" -H "X-Access-Token: <token>" http://<system>:<port>/v16/tasks -d @TestTask.json

This (and many other permutations of the "-d" option) yield:

Warning: Couldn't read data from file
Warning: "TestTask.json",

Has anybody else encountered this? My JSON appears to be good, and I can invoke the same server calls via Postman and they work using the same JSON data. It may be a Windows/cURL issue, but Google did not provide any solutions.