diff --git a/xpcom/io/nsLocalFileOS2.cpp b/xpcom/io/nsLocalFileOS2.cpp
index eacdd5ae206..313734aad53 100644
--- a/xpcom/io/nsLocalFileOS2.cpp
+++ b/xpcom/io/nsLocalFileOS2.cpp
@@ -2164,20 +2164,17 @@ nsLocalFile::IsExecutable(PRBool *_retval)
     if (pathEnd == leaf)
         return NS_OK;
 
-    // get the extension, including the dot
-    char* ext = (char*) _mbsrchr((const unsigned char*)leaf, '.');
-    if (!ext)
-        return NS_OK;
+    // get the extension, including the dot, max. of 4 chars for comparison
+    // (copy the extension so that the original filename stays untouched)
+    char ext[5];
+    strncpy(ext, (char*) _mbsrchr((const unsigned char*)leaf, '.'), 4);
+    ext[4] = '\0'; // ensure trailing nullbyte
 
     // upper-case the extension, then see if it claims to be an executable
-#ifdef MOZ_OS2_HIGH_MEMORY
     // WinUpper() cannot be used because it crashes with high memory.
     // strupr() does not take into account non-ASCII characters but this is
     // irrelevant for the possible extensions below
     strupr(ext);
-#else
-    WinUpper(0, 0, 0, ext);
-#endif
     if (strcmp(ext, ".EXE") == 0 ||
         strcmp(ext, ".CMD") == 0 ||
         strcmp(ext, ".COM") == 0 ||