Make invalid_keypress.ogg non-executable

Created by: slyon

Issue Summary

When working on the Ubuntu package for libjs-mathjax (legacy v2.7) we realized there is a potential security risk, because the file extensions/a11y/invalid_keypress.ogg is marked as executable file via its UNIX permissions.

Steps to Reproduce:

1. Clone this repository
2. Checkout the legacy-v2 branch
3. ls -la extensions/a11y/invalid_keypress.ogg

-rwxrwxr-x 1 lukas lukas 5353 Mai 22 15:41 extensions/a11y/invalid_keypress.ogg*

This audio file does not need to be executable and the +x flag should be removed, to avoid the issue where potential attackers would somehow replace it with malicious content.

chmod -x extensions/a11y/invalid_keypress.ogg

Technical details:

• MathJax Version: 2.7.4
• Client OS: Ubuntu Linux 20.04