diff --git a/mercurial/configitems.py b/mercurial/configitems.py
--- a/mercurial/configitems.py
+++ b/mercurial/configitems.py
@@ -1786,6 +1786,13 @@ coreconfigitem(
 )
 coreconfigitem(
     b'merge-tools',
+    br'.*\.regappend$',
+    default=b"",
+    generic=True,
+    priority=-1,
+)
+coreconfigitem(
+    b'merge-tools',
     br'.*\.symlink$',
     default=False,
     generic=True,
diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -158,7 +158,7 @@ def findexternaltool(ui, tool):
             continue
         p = util.lookupreg(k, _toolstr(ui, tool, b"regname"))
         if p:
-            p = procutil.findexe(p + _toolstr(ui, tool, b"regappend", b""))
+            p = procutil.findexe(p + _toolstr(ui, tool, b"regappend"))
             if p:
                 return p
     exe = _toolstr(ui, tool, b"executable", tool)