Downloading files from a site to a specific folder with a specific extension

Question

I want to implement this idea. I have a folder on my PC with the following structure: There is a Site folder and in it:

1. index.html
2. style.css
3. new folder

index.html is my site page, and I want to make a file upload button of a certain type on this page. that is, the usual, standard form of uploading files, but only for a certain type of files. And when a user uploads a file to my page, this file is placed in the New folder. How can I implement this? What should be used for this? Maybe there are articles that I missed on the Internet, although I climbed many sites. Tell me what can be done?