"How Do I Force...?"

TIP: Understand that HTML can't "force" any sort of action, and don't keep trying to get around this limitation; you'll just annoy your users and make your site less accessible.

Common "newbie" questions in the newsgroups devoted to Web authoring are those that begin "How do I force the user's browser to..."

Forcing files to download, run, launch specific applications, etc.

You can't do that. The Web protocols were designed to identify, via MIME Content-Type headers, what sort of content a data stream has, but not specify exactly what to do with it. This was done for a good reason; the site author has no way of knowing exactly what sort of system the end user has, or that user's preferences as to how to deal with different kinds of data. And some ways of dealing with data, like automatically running an .exe file, pose security risks such as viruses and "trojan horses". And if the user has a Macintosh or a Unix system, running DOS or Windows .EXEs is infeasible, anyway; but if you let the user download the file, he might be able to put it on a disk and run it on a PC down the hall.

In general, users may want to make their own choices as to how to deal with various kinds of files, displaying them in their browser, displaying them via an external helper application, or saving them to their hard disk, rather than letting your site force one particular behavior that might not even work on this particular user's system. So you should make sure your server sends an honest and accurate Content-Type header for each item it sends.

If you're sending data files of some sort which the user ought to be saving instead of viewing in his/her browser, the best MIME type to use is application/octet-stream; this will usually cause a "save" dialog box to appear. It's not "forced" (a user can configure the browser to do something else with this sort of data), but it's the usual action, and this is the closest you can get to "forcing the browser to save the file." But you can't force the directory or filename it's saved under, though browsers will generally get the name out of the last part of the URL (though they can vary in maddening ways, sometimes winding up with weird filenames unlike the one you intended).

You can always encourage the user to make use of browser features to save a file to disk, such as right-clicking in Netscape or MSIE, which work no matter what MIME type is used or how the browser is configured to handle that type.

It should also be pointed out that if the file is "forced" to disk on the user's system by means of the application/octet-stream, there may be an undesirable side effect - the user may suffer the loss of their Operating System's configured usage mappings. It may not behave as expected when it gets there! - RMR