Best practice: same button multiple functions depending on input

I have an input field where the user should enter a URL. If the url points to a sitemap (ie sitemap.xml) all the urls contained in the sitemap should be downloaded whereas in the case of a url not pointing to a sitemap, only the single url should be downloaded. I'm trying my best to keep the UI clean so I rather not mess it up with instructions etc but feel that without any instructions there is no way for the user to understand the sitemap bulk download feature. Any ideas? I did try two different buttons but felt it looked abit messy..