A reader that detects the mime/content-type in Go

content-type-reader.go

package main

import (
	"bytes"
	"io"
	"log"
	"net/http"

	"github.com/gabriel-vasile/mimetype"
)

type ContentTypeReader struct {
	buf [512]byte
	cnt   int
	done bool
	mimetype string
	r   io.Reader
}

func NewContentTypeReader(r io.Reader) *ContentTypeReader {
	return &ContentTypeReader{
		r: r,
	}
}

func (ctr *ContentTypeReader) Read(b []byte) (n int, err error) {
	n, err = ctr.r.Read(b)
	if !ctr.done {
		rem := 512-ctr.cnt
		if n >= rem {
			copy(ctr.buf[ctr.cnt:], b[:rem])
			ctr.cnt += rem
			ctr.done = true
		} else {
			copy(ctr.buf[ctr.cnt:ctr.cnt+n], b)
			ctr.cnt += n
		}
	}

	if err != nil {
		if err == io.EOF {
			// net/http.DetectContentType will show text/xml for SVGs, which is not wrong but inconvenient.
			// ctr.mimetype = http.DetectContentType(ctr.buf[:ctr.cnt])
			ctr.mimetype = mimetype.Detect(ctr.buf[:ctr.cnt]).String()
			return
		}
	}
	return
}

func (ctr *ContentTypeReader) Mimetype() string {
	return ctr.mimetype
}

func main() {
	resp, err := http.Get(`https://upload.wikimedia.org/wikipedia/commons/d/dd/Map_of_Europe_with_flags.svg`)
	if err != nil {
		log.Fatal(err)
	}
	defer resp.Body.Close()

	ctr := NewContentTypeReader(resp.Body)
	_, err = bytes.NewBuffer(nil).ReadFrom(ctr)
	if err != nil {
		log.Fatal(err)
	}
	log.Println("Mimetype:", ctr.Mimetype())
}

A more simple solution using a LimitWriter: https://gist.github.com/miku/fe4fc9f53258b23741f41c38764fd2b6