PostMultipart : have problem as there's chinese character in partsToSend

Hello, recently I use PostMultipart, and these's chinese character in my partsToSend
so I set the headers like this:

var params = {
proxyScheme: undefined /* STRING */,
headers: {"Content-type":"multipart/form-data; charset=UTF-8"} /* JSON */,
ignoreSSLErrors: undefined /* BOOLEAN */,
useNTLM: undefined /* BOOLEAN */,
partsToSend: formData /* INFOTABLE */,
workstation: undefined /* STRING */,
useProxy: undefined /* BOOLEAN */,
repository: undefined /* STRING */,
proxyHost: undefined /* STRING */,
url: url /* STRING */,
};

// result: JSON
var result = Resources["ContentLoaderFunctions"].PostMultipart(params);

Here I print out the content which api server received:

Red: If I remove the content of headers, every chinese character  will become to "?"
Orange: There's no problem as I use Postman to test the api

Green: the content which api server received will become to nothing as setting the headers

I also try to set headers: {"Content-type":"charset=Big5"} but the result is the same as UTF-8

anyone can help me?
thanks a lot!