Fix gcode.gz double extension problem on Mac

CURA-5228

plugins/GCodeGzReader/GCodeGzReader.py

@@ -3,11 +3,11 @@
 
 import gzip
 
-from io import TextIOWrapper
-
+from UM.Platform import Platform
 from UM.Mesh.MeshReader import MeshReader #The class we're extending/implementing.
 from UM.PluginRegistry import PluginRegistry
 
+
 ##  A file reader that reads gzipped g-code.
 #
 #   If you're zipping g-code, you might as well use gzip!
@@ -15,7 +15,7 @@ class GCodeGzReader(MeshReader):
 
     def __init__(self):
         super().__init__()
-        self._supported_extensions = [".gcode.gz"]
+        self._supported_extensions = [".gz"]
 
     def read(self, file_name):
         with open(file_name, "rb") as file:

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() }