Example to read a file and send it to browser with Go.

http-download.go

package main

import (
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"path/filepath"
)

func internalServerError(w http.ResponseWriter, err error) {
	log.Println(err)
	status := http.StatusInternalServerError
	w.WriteHeader(status)
	json.NewEncoder(w).Encode(map[string]interface{}{
		"status": status,
		"error":  err,
	})
}

func handleDownload(w http.ResponseWriter, r *http.Request) {
	file := "hello.txt" //your file, it can be relative path in your working dir
	filePath, err := filepath.Abs(file)
	if err != nil {
		internalServerError(w, err)
		return
	}

	f, err := os.Open(filePath)
	if err != nil {
		internalServerError(w, err)
		return
	}
	defer f.Close()

	w.Header().Set("Content-Disposition", fmt.Sprintf("attachment; filename=\"%s\"", file))
	io.Copy(w, f)
}

func main() {
	http.HandleFunc("/", handleDownload)

	http.ListenAndServe(":3000", nil)
}