GraphDB API: question about import method

I am using GraphDB 10.8 (Free), and want to use the GraphDB API to import an RDF file into the default graph. I have been reading and trying out the API documentation, but I'm failing to run the import POST method. The error seems to be the file name of the import file. I initially used the /rest/repositories/importtest/import/server GET method to show the files available for import, and that works nicely. It gives a JSON package of information that I then used to build the import POST method, but no matter what I've tried for the filenames array property, I get the error:

File [path]/briefdata.rdf does not exist

To be clear, the file does exist, at /home/ianpiper/graphdb-import (which is the default location on the server) and it is world read-write. Actually, the file is found with the GET form of the method, so it seems like in the POST method I am just not providing the right form of the path leading to the file. Can anyone set me straight on this?

Thanks,

Ian.