Fix mime-type detection for HTTP server (#18370)

Bypass the unstable behavior of Golang's mime.TypeByExtension

modules/public/serve_dynamic.go

@@ -0,0 +1,24 @@
+// Copyright 2016 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+//go:build !bindata
+// +build !bindata
+
+package public
+
+import (
+	"io"
+	"net/http"
+	"os"
+	"time"
+)
+
+func fileSystem(dir string) http.FileSystem {
+	return http.Dir(dir)
+}
+
+// serveContent serve http content
+func serveContent(w http.ResponseWriter, req *http.Request, fi os.FileInfo, modtime time.Time, content io.ReadSeeker) {
+	http.ServeContent(w, req, fi.Name(), modtime, content)
+}