[Go] Slack API を使って画像をアップロードする

package main

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

func main() {
    f, err := os.Open("./hoge.png")
    if err != nil {
        panic("error")
    }
    defer f.Close()

    bodyBuf := &bytes.Buffer{}
    writer := multipart.NewWriter(bodyBuf)
    part, err := writer.CreateFormFile("file", "some-filename")
    if err != nil {
        panic("error")
    }
    if _, err := io.Copy(part, f); err != nil {
        panic("error")
    }

    err = writer.WriteField("token", "dummy-token")
    if err != nil {
        panic("error")
    }

    err = writer.WriteField("channels", "some-channel-name")
    if err != nil {
        panic("error")
    }

    err = writer.Close()
    if err != nil {
        panic("error")
    }

    requestSlack, err := http.NewRequest(
        "POST",
        "https://slack.com/api/files.upload",
        bodyBuf)
    if err != nil {
        panic("error")
    }

    requestSlack.Header.Set("Content-Type", writer.FormDataContentType())

    clientSlack := new(http.Client)
    responseSlack, err := clientSlack.Do(requestSlack)
    if err != nil {
        panic("error")
    }
    defer responseSlack.Body.Close()
}