Fix a misalignment in the file attachment caption editor.
Review Request #6977 — Created Feb. 24, 2015 and submitted
At some point in the past, a merge from release-2.0.x to master deleted a bunch
of style rules. Christian fixed most of these, but there was one that didn't
make it, which set the box-sizing mode for file attachments. This wasn't
immediately obvious because things still looked correct, except when the
caption editor for file attachments was opened, it was 2px less wide than the
box it was supposed to be flush with.
Opened the file attachment caption editor and saw that everything aligned
nicely.