<?php
include "dbcon.php";

class file extends database {
    private $extension = ['image/png', 'image/jpeg', 'image/jpg'];
    public function upload($file) {
        if (is_array($file)) {
            if (in_array($file['type'], $this->extension)) {
                if (move_uploaded_file($file['tmp_name'], 'upload/' . $file['name'])) {
                        // $file is an array not string and it should be $file['name'] instead
                        $sql = "INSERT INTO picture (name) VALUES ('$file')";
                        if ($result = mysqli_query($this->con, $sql)) {
                            die("mysql error: " .mysqli_error($this->con)); // echo your mysql error
                        }
                } else {
                    die("something is wrong, check file and folder"); // give better description
                }
            } else {
                die("wrong file type"); // give better description
            }
        } else {
            die("file is not array"); // give better description
        }
    }
}

$fileupload = new file();

if (isset($_FILES['file'])) {
    $fileupload->upload($_FILES['file']);
} else {
  die('not submited');
}
?>