How can I set custom Content-Type headers for individual fields in a multipart form in Go?

How to Edit Content-Type Headers for Multipart Forms in Go

In Go, you may encounter the need to modify the Content-Type header for specific fields in a multipart form. While the mime/multipart package enables you to create multipart forms easily, it does not provide a direct way to set Content-Type headers for individual fields.

To overcome this limitation, you can implement a custom function like the following:

<code class="go">import (
    "mime/multipart"
    "text/template"
)

func CreateAudioFormFile(writer *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 writer.CreatePart(h)
}</code>

This function enables you to create a form field with the desired Content-Type header:

<code class="go">writer2 := multipart.NewWriter(buf)
audioFile, _ := CreateAudioFormFile(writer2, "helloWorld.wav")
io.Copy(audioFile, file)</code>

This code will generate a multipart form with the following headers:

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

Don't forget to write the file data to the field after creating it, as in the original example provided.

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