Describe the bug

After upgrading msw from 1.x to 2.x FormData upload stopped working in tests. Looking at the debugging output, Axios doesn't append the boundary to the Content-Type header anymore.

The MSW team checked [1] this already on their side.

I tried to append the boundary manually to the header for debugging purposes, but that revealed another issue:

{ "Content-Type": "multipart/form-data; boundary=abc" }

ends in

Content-Type: multipart/form-databoundary=abc

[1] mswjs/msw#1835

To Reproduce

Checkout the msw reproducer at [1] and

cd examples/with-vitest
npm install
npm test

[1] https://github.com/pschyma/msw-examples/tree/axios-formdata-reproducer

Code snippet

No response

Expected behavior

As with msw the boundary is appended to Content-Type header.

Axios Version

1.6.0

Adapter Version

XHR

Browser

jsdom

Browser Version

No response

Node.js Version

18.8.2

OS

Linux

Additional Library Versions

msw 2.0.x
vitest 0.34.6

Additional context/Screenshots

No response