imgpaste/upload.php

<?php

if(is_file('config.php')) {
	include('config.php');
} else {
	include('config.dist.php');
}

if(!is_dir($datadir)) {
	mkdir($datadir);
}

$extensions = ['jpg', 'jpeg', 'png', 'gif'];

if(isset($_FILES['upload'])) {
	$extension = pathinfo($_FILES['upload']['name'], PATHINFO_EXTENSION);
	$extension = strtolower($extension);

	if(in_array($extension, $extensions)) {
		$filename = generate_filename($length) . '.' . $extension;
		move_uploaded_file($_FILES['upload']['tmp_name'], $datadir . '/' . $filename);
		echo json_encode(['datadir' => $datadir, 'filename' => $filename]);
	}
}

function generate_filename($length) {
	$filename = "";

	for($i = 0; $i < $length; $i++) {
		$chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
		$random_char = $chars[rand(0, strlen($chars) - 1)];
		$filename = $filename . $random_char;
	}	

	return $filename;
}

?>