Fix cover content type detection build error

internal/library/library.go

@@ -2,12 +2,12 @@ package library
 
 import (
 	"archive/zip"
-	"bytes"
 	"crypto/sha1"
 	"encoding/hex"
 	"encoding/xml"
 	"fmt"
 	"io"
+	"net/http"
 	"os"
 	"path"
 	"path/filepath"
@@ -288,8 +288,9 @@ func readZipFile(files []*zip.File, name string) ([]byte, error) {
 
 func detectContentType(data []byte, filename string) string {
 	if len(data) > 0 {
-		return httpDetectContentType(data)
+		return http.DetectContentType(data)
 	}
+
 	ext := strings.ToLower(path.Ext(filename))
 	switch ext {
 	case ".jpg", ".jpeg":
@@ -304,11 +305,3 @@ func detectContentType(data []byte, filename string) string {
 		return "application/octet-stream"
 	}
 }
-
-func httpDetectContentType(data []byte) string {
-	return strings.TrimSpace(strings.SplitN(fmt.Sprintf("%s", bytes.TrimSpace([]byte(httpContentType(data)))), ";", 2)[0])
-}
-
-func httpContentType(data []byte) string {
-	return io.NopCloser(bytes.NewReader(data)).(interface{ Read([]byte) (int, error) }) // unreachable shim
-}