fix(dump): --file extension is ignored when --type is not set (#40235)

`gitea dump -f backup.tar.gz` without `--type` wrote a zip archive under
that name. Detecting the type from the file name was requested in
https://github.com/go-gitea/gitea/issues/24622 and intended in
https://github.com/go-gitea/gitea/pull/30240, but the check read the
still-empty result instead of `--file`, so every name fell back to zip.

A supported extension now selects the type, other names like `backup` or
`gitea.bak` still default to zip. An explicit `--type` still wins, and
its help text now mentions the detection.

AI disclosure: written with Claude Code (claude-opus-5-5).

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: silverwind <me@silverwind.io>

cmd/dump.go

@@ -90,7 +90,7 @@ func newDumpCommand() *cli.Command {
 			},
 			&cli.StringFlag{
 				Name:  "type",
-				Usage: `Dump output format, default to "zip", supported types: ` + strings.Join(dump.SupportedOutputTypes, ", "),
+				Usage: `Dump output format, default to the --file extension if supported, else "zip", supported types: ` + strings.Join(dump.SupportedOutputTypes, ", "),
 			},
 		},
 	}

modules/dump/dumper.go

@@ -34,15 +34,11 @@ func PrepareFileNameAndType(argFile, argType string) (outFileName, outType strin
 		outType = argType
 		outFileName = fmt.Sprintf("gitea-dump-%d.%s", timeutil.TimeStampNow(), outType)
 	} else if argType == "" {
-		if filepath.Ext(outFileName) == "" {
-			outType = SupportedOutputTypes[0]
-			outFileName = argFile
-		} else {
-			for _, t := range SupportedOutputTypes {
-				if strings.HasSuffix(argFile, "."+t) {
-					outFileName = argFile
-					outType = t
-				}
+		outFileName, outType = argFile, SupportedOutputTypes[0]
+		for _, t := range SupportedOutputTypes {
+			if strings.HasSuffix(argFile, "."+t) {
+				outType = t
+				break
 			}
 		}
 	} else {

modules/dump/dumper_test.go

@@ -47,9 +47,11 @@ func TestPrepareFileNameAndType(t *testing.T) {
 	test("a.zip", "tar.gz", "a.zip", "tar.gz")
 	test("a.zip", "no-such", "", "")
 
-	test("a.tar.gz", "", "a.tar.gz", "zip")
+	test("a.tar.gz", "", "a.tar.gz", "tar.gz")
 	test("a.tar.gz", "tar.gz", "a.tar.gz", "tar.gz")
 	test("a.tar.gz", "no-such", "", "")
+
+	test("a.b", "", "a.b", "zip")
 }
 
 func TestIsSubDir(t *testing.T) {