Summary

Specifying thumbnail dimensions in a wiki markup field (such as 'Description) no longer works. If a thumbnail is used, it will stay the same default size no matter what changes you make. The format used to change the size has always been:
Example-

!foo.jpg|thumbnail,width=40px!

Steps to Reproduce

1. Add an attachment to an issue
2. Insert a thumbnail into the description field and attempt to specify width and/or height. No changes take place.

Workaround

Use the width=, but leave the thumbnail text out. The problem though is that the thumbnail can no longer be selected/enlarged. So this workaround is not a true fix.