Can I check if a file exists, on the client's computer, on the WEB Core? I researched if I could do this with JavaScript, but found nothing.
If your question is whether there is a file local on the user computer from the browser, that is standard not possible (if it were possible, it would be a huge security violation).
Now, there is support for this coming up but of course, AFTER the user allows this via a consent dialog. This is new browser technology and explained here:
https://tmssoftwarecom.webhosting.be/site/blog.asp?post=711