File format quick reference for developers

In web application development, knowledge of file formats is frequently required for file validation, MIME type determination, and upload limit design. This article compiles a list of commonly used file formats in development. Bookmark it and use it as a quick reference you can consult at any time.

Image format

Document Format

Archive format

Text and data formats

MIME Type Validation

When implementing file upload functionality, it is important to validate not only the file extension but also the MIME type. However, since MIME type can be spoofed on the client side, also use magic byte (file header) checks on the server side.

// PHP でのマジックバイトによる判定
$finfo = new finfo(FILEINFO_MIME_TYPE);
$mimeType = $finfo->file($_FILES['upload']['tmp_name']);

$allowedTypes = [
    'image/png',
    'image/jpeg',
    'image/webp',
    'application/pdf',
];

if (!in_array($mimeType, $allowedTypes, true)) {
    throw new Exception('許可されていないファイル形式です');
}

// Node.js での file-type ライブラリを使った判定
import { fileTypeFromFile } from 'file-type';

const type = await fileTypeFromFile('upload.png');
console.log(type);
// { ext: 'png', mime: 'image/png' }

Download test files

For testing validation processing and upload functionality for each format, please use DevLab's test files.

• <a href="/ja/files/images/">Image Files</a> — PNG, JPG, WebP, GIF in various sizes
• <a href="/ja/files/pdf/">PDF Files</a> — PDF test files in various sizes
• <a href="/ja/files/zip/">ZIP Files</a> — For testing archive processing
• <a href="/ja/files/csv/">CSV files</a> — Testing data import
• <a href="/ja/files/json/">JSON Files</a> — For API testing
• <a href="/ja/files/txt/">Text Files</a> — For testing text processing
• <a href="/ja/files/broken/">Broken files</a> — Testing error handling

Summary

A correct understanding of file formats is the foundation for implementing safe and robust upload functionality. Validate using both MIME types and magic bytes, not just file extensions, to perform proper validation. We recommend bookmarking this reference so you can consult it at any time during development.