Export composed textures as JPEG unless PNG is required

addons/io_scene_gltf2/blender/exp/gltf2_blender_gather_image.py

@@ -1,4 +1,4 @@
-# Copyright 2018-2019 The glTF-Blender-IO authors.
+# Copyright 2018-2020 The glTF-Blender-IO authors.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -101,9 +101,14 @@ def __gather_mime_type(sockets_or_slots, export_image, export_settings):
 
     if export_settings["gltf_image_format"] == "AUTO":
         image = export_image.blender_image()
-        if image is not None and __is_blender_image_a_jpeg(image):
-            return "image/jpeg"
-        return "image/png"
+        if image is not None:
+            return "image/jpeg" if __is_blender_image_a_jpeg(image) else "image/png"
+        for fill in export_image.fills.values():
+            if not isinstance(fill, FillImage):
+                continue
+            if not __is_blender_image_a_jpeg(fill.image):
+                return "image/png"
+        return "image/jpeg"
 
     elif export_settings["gltf_image_format"] == "JPEG":
         return "image/jpeg"