imgbundler: Improve mime type detection

The order of priority in detecting the mime type is now:
- Content-Type response header
- File path extension
- http.DetectContentType

lib/imgbundler/imgbundler.go

@@ -6,9 +6,11 @@ import (
 	"encoding/base64"
 	"fmt"
 	"io/ioutil"
+	"mime"
 	"net/http"
 	"net/url"
 	"os"
+	"path"
 	"regexp"
 	"strings"
 	"sync"
@@ -38,7 +40,11 @@ type repl struct {
 }
 
 func bundle(ctx context.Context, ms *xmain.State, svg []byte, isRemote bool) (_ []byte, err error) {
-	defer xdefer.Errorf(&err, "failed to bundle images")
+	if isRemote {
+		defer xdefer.Errorf(&err, "failed to bundle remote images")
+	} else {
+		defer xdefer.Errorf(&err, "failed to bundle local images")
+	}
 	imgs := imageRegex.FindAllSubmatch(svg, -1)
 	imgs = filterImageElements(imgs, isRemote)
 
@@ -65,9 +71,10 @@ func bundle(ctx context.Context, ms *xmain.State, svg []byte, isRemote bool) (_
 				}()
 
 				var buf []byte
+				var mimeType string
 				var err error
 				if isRemote {
-					buf, err = httpGet(ctx, string(href))
+					buf, mimeType, err = httpGet(ctx, string(href))
 				} else {
 					buf, err = os.ReadFile(string(href))
 				}
@@ -79,7 +86,9 @@ func bundle(ctx context.Context, ms *xmain.State, svg []byte, isRemote bool) (_
 					return
 				}
 
-				mimeType := http.DetectContentType(buf)
+				if mimeType == "" {
+					mimeType = sniffMimeType(href, buf, isRemote)
+				}
 				mimeType = strings.Replace(mimeType, "text/xml", "image/svg+xml", 1)
 				b64 := base64.StdEncoding.EncodeToString(buf)
 
@@ -108,7 +117,7 @@ func bundle(ctx context.Context, ms *xmain.State, svg []byte, isRemote bool) (_
 		case repl, ok := <-replc:
 			if !ok {
 				if len(errhrefs) > 0 {
-					return svg, xerrors.Errorf("failed to bundle the following images: %v", errhrefs)
+					return svg, xerrors.Errorf("%v", errhrefs)
 				}
 				return svg, nil
 			}
@@ -141,23 +150,45 @@ func filterImageElements(imgs [][][]byte, isRemote bool) [][][]byte {
 
 var httpClient = &http.Client{}
 
-func httpGet(ctx context.Context, href string) ([]byte, error) {
+func httpGet(ctx context.Context, href string) ([]byte, string, error) {
 	ctx, cancel := context.WithTimeout(ctx, time.Minute)
 	defer cancel()
 
 	req, err := http.NewRequestWithContext(ctx, "GET", href, nil)
 	if err != nil {
-		return nil, err
+		return nil, "", err
 	}
 
 	resp, err := httpClient.Do(req)
 	if err != nil {
-		return nil, err
+		return nil, "", err
 	}
 	defer resp.Body.Close()
 	if resp.StatusCode != 200 {
-		return nil, fmt.Errorf("expected status 200 but got %d %s", resp.StatusCode, resp.Status)
+		return nil, "", fmt.Errorf("expected status 200 but got %d %s", resp.StatusCode, resp.Status)
 	}
 	r := http.MaxBytesReader(nil, resp.Body, maxImageSize)
-	return ioutil.ReadAll(r)
+	buf, err := ioutil.ReadAll(r)
+	if err != nil {
+		return nil, "", err
+	}
+	return buf, resp.Header.Get("Content-Type"), nil
+}
+
+// sniffMimeType sniffs the mime type of href based on its file extension and contents.
+func sniffMimeType(href, buf []byte, isRemote bool) string {
+	p := string(href)
+	if isRemote {
+		u, err := url.Parse(p)
+		if err != nil {
+			p = ""
+		} else {
+			p = u.Path
+		}
+	}
+	mimeType := mime.TypeByExtension(path.Ext(p))
+	if mimeType == "" {
+		mimeType = http.DetectContentType(buf)
+	}
+	return mimeType
 }