Media upload – choose only one file

WordPress contains 2 media up-loaders. The Flash uploader allows the selection of multiple files while the browser uploader only allows 1 file at a time.

To disable the Flash uploader add the following filter to functions.php

add_filter('flash_uploader', create_function('$flash', 'return false;'));

EDIT

After further investigation it’s probably not a great idea to use create_function. A beter way to remove the filter would be:

function disable_flash_uplaoder() {
        return $flash = false;
}
add_filter( 'flash_uploader', 'disable_flash_uploader', 7 );