misc: store images internally with PNG extension for easier manual tinkering

CHANGES.md

@@ -12,6 +12,7 @@ Features:
  * Add filter box to Game and Stylesheet selection.
  * Add extra_card("field name") script function for accessing Extra Card Fields.
  * Add Clear button to console panel.
+ * Store images internally with PNG extension.
 
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 HEAD: new items added as changes are made

src/gui/value/image.cpp

@@ -45,7 +45,7 @@ void ImageValueEditor::sliceImage(const Image& image) {
   // clicked ok?
   if (s.ShowModal() == wxID_OK) {
     // store the image into the set
-    LocalFileName new_image_file = getLocalPackage().newFileName(field().name,_("")); // a new unique name in the package
+    LocalFileName new_image_file = getLocalPackage().newFileName(field().name,_(".png")); // a new unique name in the package
     Image img = s.getImage();
     img.SaveFile(getLocalPackage().nameOut(new_image_file), wxBITMAP_TYPE_PNG); // always use PNG images, see #69. Disk space is cheap anyway.
     addAction(value_action(valueP(), new_image_file));