sanitize_filename_on_upload.php

<?php
function sanitize_filename_on_upload($filename) {
	$ext = end(explode('.', $filename));

	// Replace all weird characters
	$sanitized = preg_replace('/[^a-zA-Z0-9-_.]/', '_', substr($filename, 0, -(strlen($ext)+1)));

	// Replace dots inside filename
	$sanitized = str_replace('.', '-', $sanitized);

	return strtolower($sanitized . '.' . $ext);
}
add_filter('sanitize_file_name', 'sanitize_filename_on_upload', 10);
?>