Improve file content types detections

Read files and detect their types only if the identification through the extension has failed

files/file.go

@@ -139,24 +139,29 @@ func (i *FileInfo) detectType(modify, saveContent bool) error {
 	// imagine the situation where a file in a dir with thousands
 	// of files couldn't be opened: we'd have immediately
 	// a 500 even though it doesn't matter. So we just log it.
-	reader, err := i.Fs.Open(i.Path)
-	if err != nil {
-		log.Print(err)
-		i.Type = "blob"
-		return nil
-	}
-	defer reader.Close()
 
-	buffer := make([]byte, 512)
-	n, err := reader.Read(buffer)
-	if err != nil && err != io.EOF {
-		log.Print(err)
-		i.Type = "blob"
-		return nil
-	}
+	readLen := 0
+	buffer := make([]byte, 0)
 
 	mimetype := mime.TypeByExtension(i.Extension)
 	if mimetype == "" {
+		reader, err := i.Fs.Open(i.Path)
+		if err != nil {
+			log.Print(err)
+			i.Type = "blob"
+			return nil
+		}
+		defer reader.Close()
+
+		buffer = make([]byte, 512)
+		n, err := reader.Read(buffer)
+		if err != nil && err != io.EOF {
+			log.Print(err)
+			i.Type = "blob"
+			return nil
+		}
+
+		readLen = n
 		mimetype = http.DetectContentType(buffer[:n])
 	}
 
@@ -171,7 +176,7 @@ func (i *FileInfo) detectType(modify, saveContent bool) error {
 	case strings.HasPrefix(mimetype, "image"):
 		i.Type = "image"
 		return nil
-	case isBinary(buffer[:n], n) || i.Size > 10*1024*1024: // 10 MB
+	case readLen > 0 && (isBinary(buffer[:readLen], readLen) || i.Size > 10*1024*1024): // 10 MB
 		i.Type = "blob"
 		return nil
 	default: