How to Customize Content-Type for Multipart Form Fields in Go?

Customizing Content-Type for Multipart Form Fields in Go

Sending multipart forms often requires specifying a specific Content-Type header for individual fields, such as when uploading an audio file. While Go's multipart/mime package provides a convenient way to create multipart forms, it doesn't allow for setting field-specific Content-Type headers.

Solution

As there is currently no built-in support for this functionality, a custom solution can be implemented. Here's a modified version of the CreateFormFile function:

<code class="go">func CreateAudioFormFile(w *multipart.Writer, filename string) (io.Writer, error) {
    h := make(textproto.MIMEHeader)
    h.Set("Content-Disposition", fmt.Sprintf(`form-data; name="%s"; filename="%s"`, "file", filename))
    h.Set("Content-Type", "audio/wav;rate=8000")
    return w.CreatePart(h)
}</code>

Usage

To use this custom function:

<code class="go">// Get file as bytes
file, err := os.Open("helloWorld.wav")
buf := new(bytes.Buffer)
writer := multipart.NewWriter(buf)

// Create multipart form field with custom Content-Type header
audioFile, _ := CreateAudioFormFile(writer, "helloWorld.wav")

// Copy file data to multipart form field
io.Copy(audioFile, file)
writer.Close()</code>

Output

This will generate a multipart form with the following metadata:

--0c4c6b408a5a8bf7a37060e54f4febd6083fd6758fd4b3975c4e2ea93732
Content-Disposition: form-data; name="file"; filename="helloWorld.wav"
Content-Type: audio/wav;rate=8000
[file data]
--0c4c6b408a5a8bf7a37060e54f4febd6083fd6758fd4b3975c4e2ea93732--

This way, you can easily set the Content-Type for specific form fields when using the multipart/mime package.

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