If someone uploads a SVG as file attachment
Review Boards tries to generate a thumbnail.

Pillow do not support SVGs and throws an exception.

https://github.com/python-pillow/Pillow/issues/3509

 - djblets.util.templatetags.djblets_images - Error thumbnailing image file uploaded/files/2020/01/24/d6a12e49-05b7-42d9-bd6c-50985fd98f8e__check.svg and saving as uploaded/files/2020/01/24/d6a12e49-05b7-42d9-bd6c-50985fd98f8e__check_600.svg: cannot identify image file <cStringIO.StringI object at 0x7ff6a9b57b58>
Traceback (most recent call last):
  File "/opt/reviewboard/dist/lib/python2.7/site-packages/djblets/util/templatetags/djblets_images.py", line 126, in thumbnail
    image = Image.open(data)
  File "/opt/reviewboard/dist/lib/python2.7/site-packages/PIL/Image.py", line 2705, in open
    % (filename if filename else fp))
IOError: cannot identify image file <cStringIO.StringI object at 0x7ff6a9b57b58>

As SVG do not need thumbnail because Browsers can
scale that without a problem we can provide
the original file here.

Added an SVG file and saw that the thumbnail exists

instead of an error 404.