--- a/mercurial/commands.py	Thu Mar 11 15:38:35 2010 +0100
+++ b/mercurial/commands.py	Thu Mar 11 15:52:17 2010 +0100
@@ -158,8 +158,10 @@
     By default, the revision used is the parent of the working
     directory; use -r/--rev to specify a different revision.
 
-    To specify the type of archive to create, use -t/--type. Valid
-    types are:
+    The archive type is automatically detected based on file
+    extension (or override using -t/--type).
+
+    Valid types are:
 
     :``files``: a directory full of files (default)
     :``tar``:   tar archive, uncompressed
@@ -184,16 +186,32 @@
     dest = cmdutil.make_filename(repo, dest, node)
     if os.path.realpath(dest) == repo.root:
         raise util.Abort(_('repository root cannot be destination'))
-    matchfn = cmdutil.match(repo, [], opts)
-    kind = opts.get('type') or 'files'
+
+    def guess_type():
+        exttypes = {
+            'tar': ['.tar'],
+            'tbz2': ['.tbz2', '.tar.bz2'],
+            'tgz': ['.tgz', '.tar.gz'],
+            'zip': ['.zip'],
+        }
+
+        for type, extensions in exttypes.items():
+            if any(dest.endswith(ext) for ext in extensions):
+                return type
+        return None
+
+    kind = opts.get('type') or guess_type() or 'files'
     prefix = opts.get('prefix')
+
     if dest == '-':
         if kind == 'files':
             raise util.Abort(_('cannot archive plain files to stdout'))
         dest = sys.stdout
         if not prefix:
             prefix = os.path.basename(repo.root) + '-%h'
+
     prefix = cmdutil.make_filename(repo, prefix, node)
+    matchfn = cmdutil.match(repo, [], opts)
     archival.archive(repo, dest, node, kind, not opts.get('no_decode'),
                      matchfn, prefix)
 

--- a/tests/test-archive	Thu Mar 11 15:38:35 2010 +0100
+++ b/tests/test-archive	Thu Mar 11 15:52:17 2010 +0100
@@ -74,6 +74,20 @@
 hg archive -t tgz -p %b-%h test-%h.tar.gz
 gzip -dc test-$QTIP.tar.gz | tar tf - 2>/dev/null | sed "s/$QTIP/TIP/"
 
+hg archive autodetected_test.tar
+tar tf autodetected_test.tar
+
+# The '-t' should override autodetection
+hg archive -t tar autodetect_override_test.zip
+tar tf autodetect_override_test.zip
+
+for ext in tar tar.gz tgz tar.bz2 tbz2 zip; do
+    hg archive auto_test.$ext
+    if [ -d auto_test.$ext ]; then
+        echo "extension $ext was not autodetected."
+    fi
+done
+
 cat > md5comp.py <<EOF
 try:
     from hashlib import md5

--- a/tests/test-archive.out	Thu Mar 11 15:38:35 2010 +0100
+++ b/tests/test-archive.out	Thu Mar 11 15:52:17 2010 +0100
@@ -45,6 +45,14 @@
 test-TIP/bar
 test-TIP/baz/bletch
 test-TIP/foo
+autodetected_test/.hg_archival.txt
+autodetected_test/bar
+autodetected_test/baz/bletch
+autodetected_test/foo
+autodetect_override_test.zip/.hg_archival.txt
+autodetect_override_test.zip/bar
+autodetect_override_test.zip/baz/bletch
+autodetect_override_test.zip/foo
 True
 abort: archive prefix contains illegal components
 Archive:  test.zip