I was able to get the bundle to work in the main /Library/Mail/Bundles, rather than my user ~/Library, but as of a week ago (last security update?) I'm getting this-
"The QuoteFix plug-in caught an exception:
Traceback (most recent call last):
File "/Library/Mail/Bundles/QuoteFix.mailbundle/Contents/Resources/lib/python2.7/quotefix/fixer.py", line 110, in fix
File "/Library/Mail/Bundles/QuoteFix.mailbundle/Contents/Resources/lib/python2.7/quotefix/fixer.py", line 166, in remove_attachment_placeholders
TypeError: 'NoneType' object is not iterable
Please contact the developer quoting the contents of this alert."
As per this comment: