This could be problematic. When you add an <input type="file"> tag (or an <html:file> tag that renders to the same), the browser automatically handles the browse function using operating system APIs to display the browse dialogue. You have no direct control over this, and therefore can't insert any ajax calls into this process.
About the only workaround I can think of would be to write your own applet to handle the file browse and selection process and call that instead of using the <input type="file"> tag. You can then control what happens when a file is selected.