About Content-Type

Hi, My name is Costin. I don not know ifi this is teh right place for my question, but hope there's someone there to help me, even so. I want to click on a file, be it a GIF or an EXE or whatever, and want the browser to ask me to Open it or Save it to disk, like it says to unknown extensions, it cannot handle. A friend of mine told me this is about Content Type I am sending to the browser. The browser receives an extension it cannot represent itself and ask for saving or opening with the registred program. But how can I do this ? Thanks Costin