How can I upload a file using form-data that has the body specified as a string?

Question

Node.js

When I try to upload a file to mysite.com/upload like this, that site thinks the body is empty:

const FormData = require('form-data');
const form = new FormData();

form.append('file', 'file content');
form.submit('http://mysite.com', (err, res) => {
    ...
});

But if you read the file from disk through the stream, then everything is successfully loaded with the desired content:

const fs = require('fs');
const FormData = require('form-data');
const form = new FormData();

form.append('file', fs.createReadableStream('path/to/file'));
form.submit('http://mysite.com', (err, res) => {
    ...
});

Why does the first option work incorrectly?