Freeze `File.ftype` results

It is unlikely to modify these result strings.

1 files changed, 1 insertions, 1 deletions

diff --git a/file.c b/file.c
index 70c3892986..193dbea10d 100644
--- a/file.c
+++ b/file.c
@@ -2412,7 +2412,7 @@ rb_file_ftype(mode_t mode)
         t = "unknown";
     }
 
-    return rb_usascii_str_new2(t);
+    return rb_fstring_cstr(t);
 }
 
 /*