Use "application/octet-stream" if mime.TypeByExtension returns invalid type

Fixes #424

fs/operations.go

@@ -166,7 +166,7 @@ func Equal(src, dst Object) bool {
 // MimeType returns a guess at the mime type from the extension
 func MimeType(o Object) string {
 	mimeType := mime.TypeByExtension(path.Ext(o.Remote()))
-	if mimeType == "" {
+	if !strings.ContainsRune(mimeType, '/') {
 		mimeType = "application/octet-stream"
 	}
 	return mimeType