Reduce spurious logging for file attachments.
Review Request #7174 — Created April 6, 2015 and submitted
In cases where a mimetype handler or review UI rejects a given file attachment, we log the output. This is useful for knowing, for example, why the PDF review UI isn't trying to render for something which has mimetype application/pdf but is actually a .docx file. However, in this case we were also logging with `exc_info` turned on, which filled the log file with useless backtraces. This change removes those.