How to save file from web application to user's disk?

I'm working on web application which, among other things, needs to save small file to particular folder on user's hard drive whenever user clicks a button. That file will serve as input for another desktop application so it must be saved in predefined folder and predefined format. "Save as..." and save dialog would be very inconvenient. Also setting that all dow开发者_如何学Cnloads go to that folder would be troublesome.

Also, changing desktop application is not an option.

In current stage of web browsers and web standards what would be the best approach?

Is it development of custom plug in for browser? Or flash/silverlight? Something else?

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First of all this is not doable via http and html.

If this is a controlled environment maybe you could get away with simply providing a customized portable firefox to your users. Of course, all other downloads would go to that folder as well.

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Anything thats loaded from a website isn't allowed to access the computers HD, and with good reason.

That aside; Could you give it a new extension and associate a program with it that saves it in the correct place, all the user then needs to do is check 'always do this action' or something to open the file with the program when the button is clicked.

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Perhaps a different approach would be to have the desktop application retrieve the file from your web-application when it needs it - is this feasible?

This approach means you need not concern yourself with the sandboxing protection of web browsers that prevent web-applications knowing too much about the clients file-system.