Add file extension name to packed image if needed

Godot needs image extension name presented in order to recognize it. It
fix the issue on Linux platform when exporting material using image file
without extension name (it is valid on Linux).

io_scene_godot/converters/material/script_shader/node_tree.py

@@ -396,6 +396,21 @@ class AxisAlignedBoundBox:
         return aabb
 
 
+def export_image_name(image):
+    """Generate name according to Blender image name and format"""
+    if image.file_format in ('JPEG', 'JPEG2000'):
+        valid_extension_names = ('.jpg', '.jpeg')
+    else:
+        valid_extension_names = ('.' + image.file_format.lower(), )
+
+    if image.name.lower().endswith(valid_extension_names):
+        return image.name
+
+    # add extension information to image name, any one in the extension
+    # names list is valid, for simplity just choose the first one
+    return image.name + valid_extension_names[0]
+
+
 def export_texture(escn_file, export_settings, image):
     """Export texture image as an external resource"""
     resource_id = escn_file.get_external_resource(image)
@@ -403,19 +418,9 @@ def export_texture(escn_file, export_settings, image):
         return resource_id
 
     dst_dir_path = os.path.dirname(export_settings['path'])
-    dst_path = dst_dir_path + os.sep + image.name
+    dst_path = os.path.join(dst_dir_path, export_image_name(image))
 
     if image.packed_file is not None:
-        image_format = image.file_format
-        image_name_lower = image.name.lower()
-
-        if image_format in ('JPEG', 'JPEG2000'):
-            # jpg and jpeg are same thing
-            if not image_name_lower.endswith('.jpg') and \
-                    not image_name_lower.endswith('.jpeg'):
-                dst_path = dst_path + '.jpg'
-        elif not image_name_lower.endswith('.' + image_format.lower()):
-            dst_path = dst_path + '.' + image_format.lower()
         image.filepath_raw = dst_path
         image.save()
     else: