guessExtension fallback to clientExtension

[MGatner]

Description
Currently UploadedFile.php has getExtension() returning a string (not nullable) but Mimes::guessExtensionFromType() can return null, effectively causing an exception anytime a file is uploaded whose extension does not match the current Mime config list. This change causes getExtension() to return the client file's extension if Mime returns null.

I suspect this was the original intent for passing $this->getClientExtension() as the second parameter to Mimes::guessExtensionFromType(), but that acts as an arbitrator for duplicates rather than a fallback.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide