Handle file uploading with go

I've started playing with go very recently so I'm still a noob, sorry if I make too many mistakes. I've been trying to fix this for a long time but I just don't understand what's going on. In my main.go file I have a main function:

func main() {
	http.HandleFunc("/", handler)
	http.HandleFunc("/submit/", submit)
    log.Fatal(http.ListenAndServe(":8080", nil))
}

The handler function looks like this:

func handler(w http.ResponseWriter, r *http.Request) {
    data, _ := ioutil.ReadFile("web/index.html")
    w.Write(data)
}

I know this is not the best way to serve a website
The submit function looks like this:

func submit(w http.ResponseWriter, r *http.Request) {
    log.Println("METHOD IS " + r.Method + " AND CONTENT-TYPE IS " + r.Header.Get("Content-Type"))
    r.ParseMultipartForm(32 << 20)
    file, header, err := r.FormFile("uploadFile")
    if err != nil {
        json.NewEncoder(w).Encode(Response{err.Error(), true})
        return
    }
    defer file.Close()

    out, err := os.Create("/tmp/file_" + time.Now().String() + ".png")
    if err != nil {
	    json.NewEncoder(w).Encode(Response{err.Error(), true})
	    return
    }
    defer out.Close()

    _, err = io.Copy(out, file)
    if err != nil {
	    json.NewEncoder(w).Encode(Response{err.Error(), true})
	    return
    }

    json.NewEncoder(w).Encode(Response{"File '" + header.Filename + "' submited successfully", false})
}

The problem is when the submit function is executed, r.Method is GET and r.Header.Get("Content-Type") is an empty string, then it continues until the first if where r.FormFile returns the following error:
request Content-Type isn't multipart/form-data
I don't understand why r.Method is always GET and there's no Content-Type. I've tried to do the index.html in many different ways but r.Method is always GET and Content-Type is empty. Here's the function in index.html that uploads a file:

function upload() {
    var formData = new FormData();
    formData.append('uploadFile', document.querySelector('#file-input').files[0]);
    fetch('/submit', {
        method: 'post',
        headers: {
          "Content-Type": "multipart/form-data"
        },
        body: formData
    }).then(function json(response) {
        return response.json()
    }).then(function(data) {
        window.console.log('Request succeeded with JSON response', data);
    }).catch(function(error) {
        window.console.log('Request failed', error);
    });
}

And here's the HTML:

<input id="file-input" type="file" name="uploadFile" />

Note that the <input> tag is not inside a <form> tag, I thought that could be the problem so I changed both the function and the HTML to something like this:

function upload() {
    fetch('/submit', {
        method: 'post',
        headers: {
          "Content-Type": "multipart/form-data"
        },
        body: new FormData(document.querySelector('#form')
    }).then(function json(response) {
        return response.json()
    }).then(function(data) {
        window.console.log('Request succeeded with JSON response', data);
    }).catch(function(error) {
        window.console.log('Request failed', error);
    });
}

<form id="form" method="post" enctype="multipart/form-data" action="/submit"><input id="file-input" type="file" name="uploadFile" /></form>

But that didn't work neither. I've searched with Google how to use fetch() and how to receive a file upload from go and I've seen that they are pretty similar to mine, I don't know what I'm doing wrong.

UPDATE:
After using curl -v -F 'uploadFile=@\"C:/Users/raul-/Desktop/test.png\"' http://localhost:8080/submit I get the following output:

* Trying ::1...
* Connected to localhost (::1) port 8080 (#0)
> POST /submit HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.45.0
> Accept: */*
> Content-Length: 522
> Expect: 100-continue
> Content-Type: multipart/form-data; boundary=---------------------------a17d4e54fcec53f8
>
< HTTP/1.1 301 Moved Permanently
< Location: /submit/
< Date: Wed, 18 Nov 2015 14:48:38 GMT
< Content-Length: 0
< Content-Type: text/plain; charset=utf-8
* HTTP error before end of send, stop sending
<
* Closing connection 0

The console where I'm running go run main.go outputs nothing when using curl.

I managed to solve my problem, so here it is in case someone else needs it. And thanks @JiangYD for the tip of using curl to test the server.

TL;DR

• I wrote http.HandleFunc("/submit/", submit) but I was making a POST request to /submit (note the missing slash) << This is important because of redirections
• Don't specify the Content-Type yourself, the browser will do it for you

LONG ANSWER

I did as @JiangYD said and used curl to test the server, I updated my answer with the response. I found odd that there was a 301 Redirect since I didn't put it there, I decided to use the following curl command

curl -v -F 'uploadFile=@\"C:/Users/raul-/Desktop/test.png\"' -L http://localhost:8080/submit

(note the -L) That way curl followed the redirect, though it failed again because, when redirecting, curl switched from POST to GET but with that response I found out that the request to /submit was being redirected to /submit/ and I remembered that's how I wrote it in the main function.

After fixing that it still failed, the response was http: no such file and by looking at the net/http code I found that it meant the field didn't exist, so I did a quick test iterating over all the field names obtained:

for k, _ := range r.MultipartForm.File {
    log.Println(k)
}

I was getting 'uploadFile as the field name, I removed the single quotes in the curl command and now it uploaded the file perfectly

But it doesn't end here, I now knew the server was working correctly because I could upload a file using curl but when I tried uploading it through the hosted web page I got an error: no multipart boundary param in Content-Type.

So I found out I was suppose to include the boundary in the header, I changed fetch to something like this:

fetch('/submit', {
    method: 'post',
    headers: {
        "Content-Type": "multipart/form-data; boundary=------------------------" + boundary
    }, body: formData})

I calculate the boundary like this:

var boundary = Math.random().toString().substr(2);

But I still got an error: multipart: NextPart: EOF So how do you calculate the boundary? I read the spec https://html.spec.whatwg.org/multipage/forms.html#multipart/form-data-encoding-algorithm and found out the boundary is calculated by the algorithm that encodes the file, which in my case is FormData, the FormData API doesn't expose a way to get that boundary but I found out that the browser adds the Content-Type with multipart/form-data and the boundary automatically if you don't specify it so I removed the headers object from the fetch call and now it finally works!

Removing the header altogether actually works. Especially when sending the request via fetch or axios.

axios.post(
					endpoint + "/api/v1/personalslip",
					{
					  newSlip
					},
					{


					 
					}
				  )
				  .then(res => {
					console.log(res);
				  });