You are trying to solve an impossible problem. As you observed, all desktop browsers report back that they access all data types, and will gladly suck down any data that you pass to them. In cases of plug-in data, that data would start the plug in, while in others, the user might be asked where to save the data or other means. There is absolutely no way via standard means to determine whether a plug-in exists on the end computer.

The only browsers that I know of that will not accept */* mime types are some of the newer WAP browsers and a schmattering of text-to-voice ones, but nothing mainstream yet.

Now, if this plug-in that you are using has a way to delieve content that allows a user to click a link to go elsewhere (Flash, eg), then you can use that as a way to 'guard' the rest of your site; if the user has the plug-in, they'll see the short animation, and you can instruct them to "click here" to continue. Those that don't have it will have no idea where the enterance is and thus will not visit the site.

(My web ethics note: this is bad bad bad bad bad bad bad. 99% of the time, forcing the use of a plugin to delieve site contents is very very bad and not necessary. Plugin data should only be used to enhance the site's appearence and not hide content from users that can't use that plug-in. The 1% of the time where this is not the case are sites that are meant to demonstrate the abilities of these plugins. But this is my ethical opinion, and has no bearing on the tech answer above.)

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Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain