File extension management ?

How to determine if have any 'hijacked' or 'problematic' file extensions ?

Not that familiar with associated problems, only have read info like at...

http://filext.com/faq/manage_file_extensions.php.

Any other good info sources, and/or software, for help ?

Have older PC laptop with lots of programs (that use) on it, and wondering if should be checking for potential file extension problems as way of maintaining/speeding-up things ?

CCleaner also occasionally notes 'unused' file extensions, but usually dont delete since not sure if may be needed later by some program etc... how to decide ?

Thoughts ?