As per the Virtuoso Open Source 7.2.10 Release Notes support for loading JSON-LD files was added in this release, and it not supported the 7.2.9 release you indicate to be running. Thus you need to upgrade to the latest Virtuoso Open Source 7.2.11 Release , to get all the latest fixes including support for loading JSON-LD files.
Note also HowTo – Load JSON-LD files into Virtuoso.