Do not index everything whose mimetype is text/plain

We do not index the file if -
* It does not end in a .txt
* Its size is > 50 mb

We do not seem to hande large text files very well.

This is a temporary solution in order to avoid the extractor taking too
long to index a file and never finishing.

BUG: 332421
FIXED-IN: 4.13.1