Text file attachments are marked as safe text, as the thumbnailer will
escape/render the text content and wrap it in a div for display. This
works fine the first time it's rendered, and works fine with local
memory caching.

It doesn't work fine when you've rendered it, cached it, and then
retrieve it again later. The reason is that the string will start out as
safe text, but then it goes into memcached, where it gets stored as a
plain string. When it's pulled back out, it's no longer safe.

This didn't reproduce in a development environment, since Django's local
memory cache stores the object, without converting to a standard string.
It only reproduces when using something like memcached.

We now mark it as safe once we've fetched from cache. Unit tests were
added to check for these conditions.

Unit tests pass. They failed with the expected problem prior to my fix.