diff -r 2ee3ea69e8f1 -r 2167b9e3157a src/Pure/Thy/thy_header.scala
--- a/src/Pure/Thy/thy_header.scala	Fri Aug 19 16:19:59 2022 +0200
+++ b/src/Pure/Thy/thy_header.scala	Fri Aug 19 16:46:00 2022 +0200
@@ -91,7 +91,7 @@
 
   def import_name(s: String): String =
     s match {
-      case File_Name(name) if !name.endsWith(".thy") => name
+      case File_Name(name) if !File.is_thy(name) => name
       case _ => error("Malformed theory import: " + quote(s))
     }