How to Use `multipart/form-data` for File Uploads in Go?

Uploading with multipart/form-data Using Go

When working with RESTful APIs, you may encounter the need to post data with the Content-Type: multipart/form-data. This is commonly used for file uploads, where you have a combination of text and file parameters. In Go, this can be done using the mime/multipart package. Here's a solution to the issue you faced:

The issue in your code was that you were not setting the correct Content-Type header for multipart/form-data requests. Instead of using application/json, you should use multipart/form-data;charset=UTF-8.

Here's an improved version of your code that uses multipart/form-data for posting:

<code class="go">import (
    "bytes"
    "fmt"
    "io/ioutil"
    "net/http"
    "net/http/httputil"
    "net/url"
    "mime/multipart"
)

func NewPost2(url string) ([]byte, error) {
    // Create a multipart/form-data request body.
    bodyBuf := &bytes.Buffer{}
    bodyWriter := multipart.NewWriter(bodyBuf)

    // Add text parameters to the form data.
    m := make(map[string]interface{})
    m["fileName"] = "good"
    for k, v := range m {
        bodyWriter.WriteField(k, v.(string))
    }

    // Add a file parameter to the form data.
    fileBytes := []byte(Base64ToByte("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"))
    name := "test.jpg"
    fileWriter, err := bodyWriter.CreateFormFile("image_content", name)
    if err != nil {
        return nil, err
    }
    _, err = fileWriter.Write(fileBytes)
    if err != nil {
        return nil, err
    }
    bodyWriter.Close()

    // Create a new HTTP request with the multipart/form-data body.
    httpReq, err := http.NewRequest("POST", url, bodyBuf)
    if err != nil {
        return nil, err
    }
    httpReq.Header.Set("Content-Type", bodyWriter.FormDataContentType())

    // Perform the HTTP POST request.
    client := &http.Client{}
    resp, err := client.Do(httpReq)
    if err != nil {
        return nil, err
    }

    // Handle the HTTP POST response.
    defer resp.Body.Close()
    if resp.StatusCode < 200 || resp.StatusCode >= 300 {
        body, _ := httputil.DumpResponse(resp, true)
        return nil, fmt.Errorf("[%d %s]%s", resp.StatusCode, resp.Status, string(body))
    }

    // Read the HTTP response body.
    respData, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        return nil, err
    }

    // Return the HTTP response body.
    return respData, nil
}</code>

In this solution, we are creating a multipart/form-data request body using the CreateFormFile and WriteField methods provided by the mime/multipart package. We are then setting the Content-Type header accordingly. With these changes, your code should be able to successfully POST data with Content-Type: multipart/form-data.

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