POST THUMBNAIL OR FILE UPLOADING FROM THE FRONT END

form-image-submit.php

/*POST THUMBNAIL OR FILE UPLOADING FROM THE FRONT END*/
function textDomain_thumbnail_uploading_function( $file, $post_id , $set_as_featured = false ) {
	require( ABSPATH.'/wp-load.php' );
	if ( !function_exists('wp_handle_upload') ) {
		require_once(ABSPATH . 'wp-admin/includes/file.php');
	}
    $upload = wp_upload_bits( $file['name'], null, file_get_contents( $file['tmp_name'] ) );
    $wp_filetype = wp_check_filetype( basename( $upload['file'] ), null );
    $wp_upload_dir = wp_upload_dir();    
    $attachment = array(
        'guid'           => $wp_upload_dir['baseurl'] . _wp_relative_upload_path( $upload['file'] ),
        'post_mime_type' => $wp_filetype['type'],
        'post_title'     => preg_replace('/\.[^.]+$/', '', basename( $upload['file'] )),
        'post_content'   => '',
        'post_status'    => 'inherit'
    );
    $attachment_id = wp_insert_attachment( $attachment, $upload['file'], $post_id );
    require_once(ABSPATH . 'wp-admin/includes/image.php');
    $attach_data = wp_generate_attachment_metadata( $attachment_id, $upload['file'] );
    wp_update_attachment_metadata( $attachment_id, $attach_data );
    if( $set_as_featured == true ) {
    	set_post_thumbnail( $post_id, $attachment_id );
    }
    return $attachment_id;
}