browse/utils: Fix edge case in prepare_content_for_display
Contents with mimetype starting with application/ but with no detected programming language were not processed for display but they can be regular text files.
Closes #3486 (closed)
Test Plan
This was really an edge case, nevertheless I have updated tests to cover it using the same file as in the task.
Migrated from D6107 (view on Phabricator)