copy xml from received file to new file copies too much?

Question

I am receiving an XML file from an app. I have to take the content of that file and copy paste it into my own xml file. The way I do that is by GetBufferedInputStream() and copy that to the new file bestelling.xml

My POST Api:

public string Post()
    {
        var result = "";
        if (Request.Content != null)
        {
            string map = @"D:\bestelApp\bestelApp\Content\bestellingen\ontvangen";
            string count = Directory.GetFiles(map, "*.xml").Count().ToString();
            string extention = ".xml";
            string fileName = "bestelling" + (count != "0" ? count : "") + extention;

            string path = map + "//" + fileName;

            using (Stream output = File.OpenWrite(path))
            {
                using (Stream input = HttpContext.Current.Request.GetBufferedInputStream())
                {
                    input.CopyTo(output);
                }
            }


            result = new bestellingenController().ConvertXmlToObj(fileName, path);
        }
        return result;
    }

but when Copying the file's entity body to my own file, it copies more than exists in the original file.

Original file's content simply holds xml:

<xml?version=1.0>...</xml>

While the new file's content suddenly has some extra info and between that the xml:

-----------------------8d618bbd8fc0d89
Content-Disposition: form-data; name="file"; filename="14_20180912134640.xml"
Content-Type: application/octet-stream

<xml?version=1.0>...</xml>

-----------------------86d18bbd8fc0d89--

I just want the xml copied... What causes this and how do I solve this?

EDIT Looks like ChristianMurschall's hunch was correct because it IS the header that's being added into the body. but how do I exclude the header from the content?