Example PHP

<?php

$uploadDir = __DIR__ . '/uploads/';
if (!is_dir($uploadDir)) {
    mkdir($uploadDir, 0755, true);
}

if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    if (!isset($_FILES['file'])) {
        die("No file uploaded.");
    }

    $file = $_FILES['file'];
    $fileTmp = $file['tmp_name'];
    $fileName = basename($file['name']);

    $allowedMimeTypes = ['image/jpeg', 'image/png', 'image/gif'];

    if (!in_array($_SERVER['CONTENT_TYPE'], $allowedMimeTypes)) {
        die("Invalid file type.");
    }

    $destination = $uploadDir . $fileName;

    if (move_uploaded_file($fileTmp, $destination)) {
        echo "File uploaded successfully: " . htmlspecialchars($fileName);
    } else {
        echo "Failed to upload file.";
    }
}
?>

<!DOCTYPE html>
<html>
<head>
    <title>File Upload</title>
</head>
<body>
    <form action="" method="POST" enctype="multipart/form-data">
        <input type="file" name="file" required>
        <button type="submit">Upload</button>
    </form>
</body>
</html>