Post multi-part form data for audio file - go

Question

I'm wrote a function that posts multipart form data to an endpoint but doesn't seem to be working properly. Here's the code
func Upload(filePath string) error {
client := &http.Client{
Timeout: time.Second * 10,
}
// New multipart writer.
body := &bytes.Buffer{}
file, err := os.Open(filePath)
if err != nil {
return err
}
writer := multipart.NewWriter(body)
fw, err := writer.CreateFormFile("file", filepath.Base(filePath))
if err != nil {
return err
}
_, err = io.Copy(fw, bufio.NewReader(file))
if err != nil {
return err
}
// Close multipart writer.
writer.Close()
req, err := http.NewRequest("POST", "http://localhost:5050/upload", bytes.NewReader(body.Bytes()))
if err != nil {
return err
}
req.Header.Set("Content-Type", writer.FormDataContentType())
rsp, _ := client.Do(req)
if rsp == nil {
return fmt.Errorf("failed to upload")
}
if rsp.StatusCode != http.StatusOK {
fmt.Printf("Request failed with response code: %d", rsp.StatusCode)
}
fmt.Println("rsp: ", rsp.StatusCode)
return nil
}
The handler on the API receiving the POST is this:
func (m *MetadataService) uploadHandler(res http.ResponseWriter, req *http.Request) {
file, handler, err := req.FormFile("file")
if err != nil {
panic(err) //dont do this
}
defer file.Close()
fmt.Println("req: ", req)
// Create a buffer to store the header of the file in
fileHeader := make([]byte, 512)
// Copy the headers into the FileHeader buffer
if _, err := file.Read(fileHeader); err != nil {
panic(err) //dont do this
}
// set position back to start.
if _, err := file.Seek(0, 0); err != nil {
panic(err) //dont do this
}
// copy example
f, err := os.OpenFile(handler.Filename, os.O_WRONLY|os.O_CREATE, 0666)
if err != nil {
panic(err) //please dont
}
defer f.Close()
buf := bytes.NewBuffer(nil)
if _, err := io.Copy(buf, file); err != nil {
panic(err)
}
fmt.Println("filename:", handler.Filename)
fmt.Println("file.(Sizer).Size():", file.(Sizer).Size())
fmt.Println("contentType:", http.DetectContentType(fileHeader))
io.WriteString(res, id)
}
The code panics when reading the fileHeader thought. The content type when sent is "application/octet-stream" when I'd expect it to be "audio/mpeg" since i'm uploading a song. Not sure what's wrong with the aforementioned Upload function. Need help! Thank you!