fix text file preview in rails3

test plan:
 - upload a text file to a course
 - add it to a module
 - click the module item
 - you should see the file content,
   not "<#Proc ...>"

fixes CNVS-11934

Change-Id: Ieb3817365306080ddbb98664c62e61eea9882f72
Reviewed-on: https://gerrit.instructure.com/32162
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>