How to Detect File MIME Type in Node.js: Reliable Methods & Examples

Learn how to get the MIME type of files in Node.js using the mime-types, path, and file-type packages. Includes code examples for static and dynamic detection.

📄 How to Detect File MIME Type in Node.js: Reliable Methods & Examples

When building file upload services, APIs, or static file servers in Node.js, it’s important to accurately determine a file’s MIME type. The MIME type tells browsers or APIs what kind of content to expect — like "text/html" for HTML files or "image/png" for PNG images.

In this blog, you’ll learn:

• What a MIME type is
• How to detect MIME types using file extensions
• How to detect MIME types using file content (buffer)
• Best libraries for MIME detection in Node.js

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📚 What Is a MIME Type?

MIME (Multipurpose Internet Mail Extensions) types describe the nature and format of a file. For example:

File Extension	MIME Type
.html	text/html
.png	image/png
.pdf	application/pdf

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🔌 Method 1: Detect MIME Type by File Extension (using mime-types)

Install the mime-types package:

npm install mime-types

Usage:

const mime = require('mime-types');

const filePath = 'example.pdf';
const mimeType = mime.lookup(filePath);

console.log(`MIME type: ${mimeType}`); // application/pdf

✅ Best for static servers and known file types
⚠️ Not secure for validating untrusted uploads

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📎 Method 2: Detect MIME Type from Buffer (using file-type)

If you need to inspect the actual file content, use file-type.

Install:

npm install file-type

Usage:

const fs = require('fs');
const { fileTypeFromFile } = require('file-type');

async function detectMimeType(filePath) {
  const result = await fileTypeFromFile(filePath);
  console.log(result); // { ext: 'png', mime: 'image/png' }
}

detectMimeType('image.png');

✅ Secure and accurate
✅ Works even if file extension is wrong
⚠️ Slightly slower due to content inspection

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🆚 mime-types vs file-type

Feature	mime-types	file-type
Based on file extension	✅ Yes	❌ No
Based on file content	❌ No	✅ Yes
Ideal for trusted files	✅ Yes	✅ Yes
Ideal for untrusted input	❌ No	✅ Yes

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🛠 Bonus: Combine Both for Safety

A hybrid approach for safer validation:

const mime = require('mime-types');
const { fileTypeFromFile } = require('file-type');

async function safeMimeCheck(filePath) {
  const extMime = mime.lookup(filePath);
  const contentMime = await fileTypeFromFile(filePath);

  console.log('From extension:', extMime);
  console.log('From content:', contentMime?.mime);
}

🔐 Use Cases

• File upload validation in APIs
• Building a file previewer
• Preventing malicious uploads (e.g., disguised .exe)
• Serving correct headers in Express static file servers

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📦 Related Libraries

• mime-types – Lookup by extension
• file-type – Detect from buffer
• path – Extract extensions

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🧪 Conclusion

Detecting MIME types in Node.js is easy with the right tools. Use:

• mime-types for fast lookups from file extensions.
• file-type for reliable MIME detection from actual content.

For secure applications, especially where user uploads are involved, always validate based on file content — not just extensions.

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