总之，你需要使用mime/multipart package来构建表单。

package sample

import (
    "bytes"
    "fmt"
    "io"
    "mime/multipart"
    "net/http"
    "os"
)

func Upload(url,file string) (err error) {
    // Prepare a form that you will submit to that URL.
    var b bytes.Buffer
    w := multipart.NewWriter(&b)
    // Add your image file
    f,err := os.Open(file)
    if err != nil {
        return 
    }
    defer f.Close()
    fw,err := w.CreateFormFile("image",file)
    if err != nil {
        return 
    }
    if _,err = io.Copy(fw,f); err != nil {
        return
    }
    // Add the other fields
    if fw,err = w.CreateFormField("key"); err != nil {
        return
    }
    if _,err = fw.Write([]byte("KEY")); err != nil {
        return
    }
    // Don't forget to close the multipart writer.
    // If you don't close it,your request will be missing the terminating boundary.
    w.Close()

    // Now that you have a form,you can submit it to your handler.
    req,err := http.NewRequest("POST",url,&b)
    if err != nil {
        return 
    }
    // Don't forget to set the content type,this will contain the boundary.
    req.Header.Set("Content-Type",w.FormDataContentType())

    // Submit the request
    client := &http.Client{}
    res,err := client.Do(req)
    if err != nil {
        return 
    }

    // Check the response
    if res.StatusCode != http.StatusOK {
        err = fmt.Errorf("bad status: %s",res.Status)
    }
    return
}