how to filter file extension when user select file to upload to a case?

I had found a page as below

https://support.pega.com/question/how-do-i-limit-file-extensions-upon-uploading-file

However what's expected is to add filter when user select file instead of post validation after upload, pls refer to below, as-is filter only *.*, expect add more e.g. *.pdf, *.txt