[5.2] Refactor all instances of File::getExt() & stripExt() to use framework

[Hackwar]

Summary of Changes

We are in the process of migrating away from the CMS filesystem package to use the framework version of it. This PR converts all instances where File::getExt() & stripExt() is used to use the framework version.

Testing Instructions

Codereview.

Link to documentations

Please select:

• [ ] Documentation link for docs.joomla.org:

• [X] No documentation changes for docs.joomla.org needed

• [ ] Pull Request link for manual.joomla.org:

• [X] No documentation changes for manual.joomla.org needed

[HLeithner]

No testing instructions, touches several places of the test is untestable, better make prs for the same area and not per type.

[HLeithner]

thanks, merged by code review, both functions are identical between framework and cms version of filesystem\File and no other usages found in the changed files for this classes.