Only open USB ports if the printer understands gcode.

plugins/USBPrinting/USBPrinterOutputDeviceManager.py

@@ -65,10 +65,11 @@ class USBPrinterOutputDeviceManager(QObject, OutputDevicePlugin):
             if container_stack is None:
                 time.sleep(5)
                 continue
+            port_list = []  # Just an empty list; all USB devices will be removed.
             if container_stack.getMetaDataEntry("supports_usb_connection"):
-                port_list = self.getSerialPortList(only_list_usb=True)
-            else:
-                port_list = []  # Just use an empty list; all USB devices will be removed.
+                machine_file_formats = [file_type.strip() for file_type in container_stack.getMetaDataEntry("file_formats").split(";")]
+                if "text/x-gcode" in machine_file_formats:
+                    port_list = self.getSerialPortList(only_list_usb=True)
             self._addRemovePorts(port_list)
             time.sleep(5)