Generally I would agree with @HelioGuilherme66 on this, there really should be an input of type file hidden somewhere in the html page.
I have however once seen an exception to this where the upload button actually triggered a file selector from javascript and then did the upload via a webservice call and once successful used used a callback to update the page to show the filename as uploaded. Now this is NOT standard html or standard anything really but I did see it once.
I suggest you load the page, open dev tools (typically F12) to the network tab, clear the network tab and then upload a small file and see what network traffic is generated, inspect the post data of all your posts, and this should give you a clue to how the files are being uploaded.
Once you know what the browser does manually then you can plan an approach to automate it.
Hope this helps,
Dave.