Restricting users from uploading all file types except one

Hay,
I'm guessing your talking about MOSS.
In MOSS there are synchronous Event Handlers that you can apply to document libraries (or lists), and in them you can check the type of file the user tries to upload.
That is the only option of restriction I can think of.

There is also a way to let a user choose from a predetermined content types, and with a feature remove the ability to upload files, but then you should supply a document template and the user won't be able to upload already written files.
And if the application has the ability to save files on web sites (like all new office programs), the user will be able to upload the files directly to the site.

I think your best bet is event handlers.

Hope I was helpful.