Adapted from https://pkg.go.dev/net/http#example-FileServer-DotFileHiding

videoFileHost.go

package main

import (
	"io/fs"
	"log"
	"net/http"
	"strings"
)

func isVideoFile(name string) bool {
	if strings.HasSuffix(name, ".mp4") {
		return true
	}
	return strings.HasSuffix(name, ".mkv")
}

type videoFile struct {
	http.File
}

func (f videoFile) Readdir(n int) (fis []fs.FileInfo, err error) {
	files, err := f.File.Readdir(n)
	for _, file := range files {
		if isVideoFile(file.Name()) {
			fis = append(fis, file)
		}
	}
	return
}

type videoFileFS struct {
	http.FileSystem
}

func (fsys videoFileFS) Open(name string) (http.File, error) {
	if !(name == "/" || isVideoFile(name)) {
		return nil, fs.ErrPermission
	}

	file, err := fsys.FileSystem.Open(name)
	if err != nil {
		return nil, err
	}
	log.Println("open", name)
	return videoFile{file}, err
}

func main() {
	fsys := videoFileFS{http.Dir(".")}
	log.Fatal(http.ListenAndServe(":8080", http.FileServer(fsys)))
}