Fix gcode.gz double extension problem on Mac

CURA-5228

plugins/GCodeGzReader/__init__.py

@@ -1,21 +1,24 @@
 # Copyright (c) 2018 Ultimaker B.V.
 # Cura is released under the terms of the LGPLv3 or higher.
 
+from UM.i18n import i18nCatalog
+from UM.Platform import Platform
+
 from . import GCodeGzReader
 
-from UM.i18n import i18nCatalog
 i18n_catalog = i18nCatalog("cura")
 
 def getMetaData():
+    file_extension = "gz" if Platform.isOSX() else "gcode.gz"
     return {
         "mesh_reader": [
             {
-                "extension": "gcode.gz",
+                "extension": file_extension,
                 "description": i18n_catalog.i18nc("@item:inlistbox", "Compressed G-code File")
             }
         ]
     }
 
 def register(app):
-    app.addNonSliceableExtension(".gcode.gz")
+    app.addNonSliceableExtension(".gz")
     return { "mesh_reader": GCodeGzReader.GCodeGzReader() }