Describe the bug

Passing a FormData object to Axios client may result in request header 'Content-Length': 'NaN' being set.

To Reproduce

Given the following code:

// client is an Axios instance
const pdf = await client.get(pdfUrl, { responseType: 'stream' });

const form = new FormData();
form.append('file', pdf.data, { filename, contentType: 'application/pdf' });

const res = await client.post(urlToUploadService, form);

We get the following request header added:

    headers: AxiosHeaders {
      Accept: 'application/json, text/plain, */*',
      'Content-Type': 'multipart/form-data; boundary=--------------------------852062351748726458865935',
      'User-Agent': 'axios/1.3.2',
      'Content-Length': 'NaN',
      'Accept-Encoding': 'gzip, compress, deflate, br'
    },

I believe 'Content-Length': 'NaN', is not something we want :) I'm pretty sure the header was not added with Axios version 1.2.6.

I believe this is related code:

If we log form's getLength:

  form.getLength((err, length) => {
    console.log(err, length);
  }),

We'll get a hint of what is going on:

null NaN

Axios Version

1.3.2

Adapter Version

No response

Browser

No response

Browser Version

No response

Node.js Version

v18.14.0

OS

macos 13.2

Additional Library Versions

No response

Additional context/Screenshots

• I believe fix(formdata): add hotfix to use the asynchronous API to compute the content-length header value; #5521 is relevant as it tried to fix some related issue(?).
• Not 100% sure if this is a https://github.com/form-data/form-data issue or an Axios issue, but given it previously worked I though I would register it here.