How to handle and process multipart/form-data submissions from HTML forms in Go?

To handle and process multipart/form-data submissions from HTML forms in Go, you can use the multipart package from the standard library. Follow these steps:

Import the necessary packages:

import (
    "mime/multipart"
    "net/http"
)

In your HTTP handler, check the request method to ensure it's a POST request, and set the appropriate maximum request size and parsing limits:

func handleSubmit(w http.ResponseWriter, r *http.Request) {
    if r.Method != http.MethodPost {
        http.Error(w, "Method not allowed", http.StatusMethodNotAllowed)
        return
    }

    r.ParseMultipartForm(32 << 20) // Set the maximum request size to 32MB
}

Access the multipart/form-data form fields using r.FormValue() or r.PostFormValue(). For example, to retrieve the value of an input field with a name attribute of "name":

func handleSubmit(w http.ResponseWriter, r *http.Request) {
    // ...

    name := r.FormValue("name")
    // process the value of the "name" field
}

Access the uploaded files using r.FormFile(). This function returns a multipart.File and a multipart.FileHeader. Read the file using the Read() method, and process it as needed. Make sure to close the file when finished using it:

func handleSubmit(w http.ResponseWriter, r *http.Request) {
    // ...

    file, header, err := r.FormFile("file")
    if err != nil {
        // handle the error
    }
    defer file.Close()

    // process the file
    // e.g., save it to disk, extract metadata, etc.
}

That's it! You can now handle and process multipart/form-data submissions from HTML forms in Go.