fetch - Missing boundary in multipart/form-data POST

I want to send a new FormData() as the body of a POST request using the fetch api

The operation looks something 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"
    },
    body: formData
  }
)

The problem here is that the boundary, something like

boundary=----WebKitFormBoundaryyEmKNDsBKjB7QEqu

never makes it into the Content-Type: header

It should look like this:

Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryyEmKNDsBKjB7QEqu

When you try the "same" operation with a new XMLHttpRequest(), like so:

var request = new XMLHttpRequest()
request.open("POST", "https://api.mything.com")
request.withCredentials = true
request.send(formData)

the headers are correctly set

Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryyEmKNDsBKjB7QEqu

So my questions are:

1. how do I make fetch behave exactly like XMLHttpRequest in this situation?
2. if this is not possible, why?

Thanks everybody! This community is more or less the reason I have professional success.

The solution to the problem is to explicitly set Content-Type to undefined so that your browser or whatever client you're using can set it and add that boundary value in there for you. Disappointing but true.