serve dlna: fix missing mime types on Android causing missing videos

Before this fix serve dlna was only using the built in database of
mime types to look up the mime types of files.  On Android (and
possibly other systems) this is very small.

The symptoms of this problem was serve dlna only listing images and
not videos.

After this fix we use the backend's idea of the mime type if possible
which will be more accurate.

Fixes #3475

cmd/serve/dlna/cds.go

@@ -1,6 +1,7 @@
 package dlna
 
 import (
+	"context"
 	"encoding/xml"
 	"fmt"
 	"log"
@@ -33,7 +34,7 @@ var mediaMimeTypeRegexp = regexp.MustCompile("^(video|audio|image)/")
 
 // Turns the given entry and DMS host into a UPnP object. A nil object is
 // returned if the entry is not of interest.
-func (cds *contentDirectoryService) cdsObjectToUpnpavObject(cdsObject object, fileInfo os.FileInfo, host string) (ret interface{}, err error) {
+func (cds *contentDirectoryService) cdsObjectToUpnpavObject(cdsObject object, fileInfo vfs.Node, host string) (ret interface{}, err error) {
 	obj := upnpav.Object{
 		ID:         cdsObject.ID(),
 		Restricted: 1,
@@ -51,7 +52,15 @@ func (cds *contentDirectoryService) cdsObjectToUpnpavObject(cdsObject object, fi
 		return
 	}
 
-	mimeType := fs.MimeTypeFromName(fileInfo.Name())
+	// Read the mime type from the fs.Object if possible,
+	// otherwise fall back to working out what it is from the file path.
+	var mimeType string
+	if o, ok := fileInfo.DirEntry().(fs.Object); ok {
+		mimeType = fs.MimeType(context.TODO(), o)
+	} else {
+		mimeType = fs.MimeTypeFromName(fileInfo.Name())
+	}
+
 	mediaType := mediaMimeTypeRegexp.FindStringSubmatch(mimeType)
 	if mediaType == nil {
 		return