I have done a few tests, two approaches I have used as below
(1) Download this zipped file
(2) Download this photo
(3) Download this photo
Click (1) above the browser will ask me to download file. For (2) the image file will be opened by the browser without me being asked to save or open the file by application. I have repeated this test using a pdf file, again I couldn't stop the broswer from opening it even if I have renamed the file with a funny extension other than "pdf". I guess I have to look up a HTML book to find out why.