The YAML field spec should be provided in one argument exactly and be a valid YAML document:
yakut call 4 uavcan.file.read '[0, "/file"]'
You can also specify the fields manually:
yakut call 4 uavcan.file.read '{offset: 0, path: "/file"}'
or even:
yakut call 4 uavcan.file.read '{offset: 0, path: {path: "/file"}}'