Steps to reproduce the issue

Download a system info .txt file from the backend

Expected result

.txt file is downloaded with standard text/plain MIME type

Actual result

.txt file is downloaded with text/x-c++ MIME (at least in my case, haven't had a chance to push the RC onto another server for testing)

System information (as much as possible)

PHP Built On Darwin Michaels-MacBook-Pro.local 14.5.0 Darwin Kernel Version 14.5.0: Tue Sep 1 21:23:09 PDT 2015; root:xnu-2782.50.1~1/RELEASE_X86_64 x86_64
Database Version 5.6.29
Database Collation utf8mb4_general_ci
Database Connection Collation utf8mb4_general_ci
PHP Version 7.0.4
Web Server Apache/2.4.16 (Unix) PHP/7.0.4 OpenSSL/0.9.8zg
WebServer to PHP Interface apache2handler
Joomla! Version Joomla! 3.5.0-rc Release Candidate [ Ember ] 10-March-2016 00:30 GMT (staging branch @ 06ea85a)
Joomla! Platform Version Joomla Platform 13.1.0 Stable [ Curiosity ] 24-Apr-2013 00:00 GMT
User Agent Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/49.0.2623.87 Safari/537.36

Additional comments

Also see joomla/jissues#760. Because we have additional file validations in place, .txt files not matching the "normal" text/plain MIME fail validation when being uploaded into the issue tracker which prevents these files from being attached through that interface. This is also a PHP environmental quirk as some environments seem to convert it to the right MIME anyway.