I'd recommend to add the following file extensions to the $upload_extensions_allow array by default:
and maybe also 'docx','pptx','xlsx' alsthough I advise people not to upload those formats, some want to.
There is another issue with file extensioins which I'm uncertain if it's so easy to fix.
Most CKEditor plugins include .js and/or .html files. Installation will fail unless one allows these file formats in gpconfig.php.
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