wp-upload.php

require_once( ABSPATH . 'wp-admin/includes/file.php' );
require_once( ABSPATH . 'wp-admin/includes/image.php' );

$uploadedfile = $_FILES['file'];
$upload_overrides = array( 'test_form' => false );
$movefile = wp_handle_upload( $uploadedfile, $upload_overrides );
if ( $movefile ) {
    $wp_filetype = $movefile['type'];
    $filename = $movefile['file'];
    $wp_upload_dir = wp_upload_dir();
    $attachment = array(
        'guid' => $wp_upload_dir['url'] . '/' . basename( $filename ),
        'post_mime_type' => $wp_filetype,
        'post_title' => preg_replace('/\.[^.]+$/', '', basename($filename)),
        'post_content' => '',
        'post_status' => 'inherit'
    );
    //$post_id (the post you want this image to be attached with)
    $attach_id = wp_insert_attachment( $attachment, $filename, $post_id);
    
    // Generate the metadata for the attachment, and update the database record.
    $attach_data = wp_generate_attachment_metadata( $attach_id, $filename );
    wp_update_attachment_metadata( $attach_id, $attach_data );
} else {
    echo "Possible file upload attack!\n";
}