Fix an exception in the API when accessing file attachments without review UIs.
Fix an exception in the API when accessing file attachments without review UIs.

The thumbnail field in FileAttachment was accessing the review_ui without first
checking it for None-ness. Trivial fix.

Fixes bug 3145.

Reviewed at https://reviews.reviewboard.org/r/5032/

reviewboard/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,custode/reviewboard,KnowNo/reviewboard,brennie/reviewboard,brennie/reviewboard,custode/reviewboard,chipx86/reviewboard,1tush/reviewboard,chipx86/reviewboard,beol/reviewboard,custode/reviewboard,custode/reviewboard,beol/reviewboard,sgallagher/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,1tush/reviewboard,1tush/reviewboard,1tush/reviewboard,KnowNo/reviewboard,beol/reviewboard,chipx86/reviewboard,davidt/reviewboard,bkochendorfer/reviewboard,1tush/reviewboard,beol/reviewboard,brennie/reviewboard,sgallagher/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,reviewboard/reviewboard,KnowNo/reviewboard,bkochendorfer/reviewboard,davidt/reviewboard,1tush/reviewboard,sgallagher/reviewboard,1tush/reviewboard,brennie/reviewboard,KnowNo/reviewboard,reviewboard/reviewboard,1tush/reviewboard,chipx86/reviewboard,1tush/reviewboard