How we can upload our own javascript and HTML files
Webflow isn’t designed to upload and host custom files.
You design your site in Webflow, and you integrate your custom code through embeds or the custom code area.
If you want to maintain discrete JS files, you can host them externally, e.g. Netlify.
HTML, you could do the same but you’d need to IFRAME it or fetch it with custom code for the browser to render it as part of your Webflow pages.