How to attach a file without the user being able to explore all files?

We have a series of files that are categories & have a system where a user can create an advert, but the only way we can see to attach an advert into the category is by using HTML input type="file" however that gives the user access to all the folders & files in the computer where we would prefer to limit the users access to only files in Adverts folder & under his/her username. We have tried a few snippets of HTML coding to try such limitation but nothing has worked.

Even if we could get some HTML coding to work we do realize that HTML is frontend, similar to Javascript, & suspect that a clever user could manipulate the HTML coding stored in "his/her own browser" & gain access to whatever is desired. We dont want that.

Is there any PHP backend coding that will restrict a user to say the Adverts folder & under his/her username?