Mezzanine thumbnail templatetag failing to save thumbnails of jpeg images with Pillow >= 4.2.0

[brunobbbs]

Hello,

I've the following environment:

• Mezzanine 4.2.3
• Django 1.10.7
• Python 3.6.1
• Pillow 4.2.1

This version of Mezzanine installed the Pillow above.

Before Pillow 4.2.0, attempting to save an RGBA image as JPEG would discard the alpha channel. From Pillow 3.4.0, a deprecation warning was shown. From Pillow 4.2.0, the deprecation warning is removed and an IOError is raised.

This deprecation was discussed on Pillow Issue 2609

This makes that the thumbnail of jpeg images doesn't be saved.

To fix this, my suggestion is remove the RGB to RGBA conversion on the line 390 if the image's filetype was JPG or JPEG as follow:

if image.mode not in ("P", "L", "RGBA") and filetype not in ("JPG", "JPEG"):

The Alpha channel for JPG/JPEG images is unnecessary since these images doesn't have transparency.