This week's book giveaway is in the Mac OS forum. We're giving away four copies of a choice of "Take Control of Upgrading to Yosemite" or "Take Control of Automating Your Mac" and have Joe Kissell on-line! See this thread for details.
If the client has an application registered with the browser to handle csv files (like Excel) you can use the "inline" keyword with the Content-Disposition tag to have the browser open the document in an embedded instance of that application.
If the browser doesn't have anything registered, then the only thing it can do is prompt the user for more instruction.