通过Go中的Multipart / FormData从上传的文件中删除边界和标头

Question

I am trying to upload a file with multipart/form-data using Insomnia REST client. But uploaded files has headers.

File content is:

<?xml version="1.0" encoding="utf-8"?>
<PresetGroup xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Presets>
  ...

And my code that handles upload multipart:

if err = r.ParseMultipartForm(32 << 20); nil != err {
    http.Error(w, err.Error(), http.StatusInternalServerError)
    return
}
r.ParseMultipartForm(32 << 20)
file, _, err := r.FormFile("file")
if err != nil {
    http.Error(w, err.Error(), http.StatusInternalServerError)
    return
}
defer file.Close()
reader = io.Reader(file)

Now, when I handle reader and write to a file, File becomes this:

--X-INSOMNIA-BOUNDARY
Content-Disposition: form-data; name="file"; filename="presets.xml" Content-Type: application/octet-stream

<?xml version="1.0" encoding="utf-8"?>
<PresetGroup xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <Presets>
  ...

Why content-type and boundary added (prepended) to file? How can I avoid it?