How do I fix: Method 2: Using `FormData` with `Append()` method?

Step 1: Use the `append()` method to add a field to the form data, like this:

Coding⏱️ 2 min read📅 2026-05-31

How to Fix: fetch - Missing boundary in multipart/form-data POST

Q: Best Solutions to Fix ItMethod 1: Adding Boundary Parameter?

Step 1: Include a boundary parameter in your `Content-Type` header, like this:

Missing boundary in multipart/form-data POST request using fetch API

Quick Answer: Add the 'boundary' property to the headers object with a unique value, e.g. 'boundary=---------------------------boundary'.

📋 Table of Contents

✅ Best Solutions to Fix It
🎯 Final Words

To fix the 'Missing boundary in multipart/form-data POST' error, you need to include a boundary parameter in your `Content-Type` header.

✅ Best Solutions to Fix It

Method 1: Adding Boundary Parameter

Step 1: Include a boundary parameter in your `Content-Type` header, like this:

fetch('https://api.myapp.com', {
  method: 'POST,
  headers: {
    "Content-Type": "multipart/form-data; boundary=1234567890"
  },
  body: formData
})

This will tell the server to use a specific boundary when parsing the multipart/form-data request.

Method 2: Using `FormData` with `Append()` method

Step 1: Use the `append()` method to add a field to the form data, like this:

var formData = new FormData();
formData.append('myfile', file, 'someFileName.csv');
fetch('https://api.myapp.com', {
  method: 'POST,
  headers: {
    "Content-Type": "multipart/form-data;
      boundary=1234567890"
  },
  body: formData
}))

This will also tell the server to use a specific boundary when parsing the multipart/form-data request.

🎯 Final Words

By following these methods, you should be able to fix the 'Missing boundary in multipart/form-data POST' error and successfully send your form data using the Fetch API.

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