How to limit possible extensions to be loaded by the client in JS?

Question

JavaScript

The site has

How to make it so that on the html page it is possible to load files only with JSON extensions?

Answer 1

On the client, we do not intervene in the file, so only the server can really filter out invalid files.
Therefore, on the client, you do not allow files to be uploaded by extensions (filter), and on the server you do the rest of the validation.