wphd-filter-wphd_file_upload.php

<?php

// Assign a filter
add_filter( 'wphd_file_upload', 'my_wphd_file_upload' );

/**
 * Check if uploaded file extension is valid.
 * 
 * This function is executed using 'wphd_file_upload' filter,
 * it stops file upload using die() if incorrect file type is sent
 * 
 * @param array $file Element of $_FILES array
 * @return array
 */ 
function my_wphd_file_upload( $file ) {
    $allowed = array("pdf", "zip", "jpeg", "jpg", "gif", "png", "doc", "docx");
    $ext = pathinfo($file["name"], PATHINFO_EXTENSION);
        
    if( ! in_array( $ext, $allowed ) ) {
        $response = new stdClass();
        $response->result = 0;
        $response->msg = sprintf("Uploading file wit extension '%s' is not allowed.", $ext);
        die(json_encode($response));
    }
        
    return $file;
}