image: fix export of paletted PNGs to jpeg

 * Fix rendering of some PNGs that could not be converted to jpeg.
   Paletted pngs can't be directly converted to jpeg, they need to be
   converted to RGB mode first.  The conversion can be ugly, so a
   better approach will need to be found, but this will do for now.

Author: felliott

mfr/extensions/image/export.py

@@ -44,6 +44,13 @@ def export(self):
                     image = image.resize((round(image.size[0] * ratio),
                                           round(image.size[1] * ratio)), Image.ANTIALIAS)
 
+            # Mode 'P' is for paletted images. They must be converted to RGB before exporting to
+            # jpeg, otherwise Pillow will throw an error.  This is a temporary workaround, as the
+            # conversion is imprecise and can be ugly.
+            # See: https://stackoverflow.com/q/21669657
+            if image.mode == 'P':
+                image = image.convert('RGB')
+
             # handle transparency
             # from https://github.com/python-pillow/Pillow/issues/2609
             if image.mode in ('RGBA', 'RGBa', 'LA') and type in ['jpeg', 'jpg']: