Fix rendering of text file thumbnails for unopened files.
Fix rendering of text file thumbnails for unopened files.

Text file thumbnails, when accessed via the API, didn't have access to
an opened file. This was causing an error when attempting to read the
file data. The solution is just to open the file before doing any
reading from it.

Fixes bug #2805

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