diff --git a/blogposts/blogposts.go b/blogposts/blogposts.go
index f959fc9..bd41183 100644
--- a/blogposts/blogposts.go
+++ b/blogposts/blogposts.go
@@ -1,9 +1,17 @@
 package blogposts
 
 import (
+	"errors"
 	"io/fs"
+	"strings"
 )
 
+var markdownSuffix = map[string]struct{}{
+	"md": {},
+}
+
+var ErrUnknownFileType = errors.New("unknown file type, must be markdown")
+
 func NewPostsFromFS(fileSystem fs.FS) ([]Post, error) {
 	dir, err := fs.ReadDir(fileSystem, ".")
 	if err != nil {
@@ -11,7 +19,11 @@ func NewPostsFromFS(fileSystem fs.FS) ([]Post, error) {
 	}
 	var posts []Post
 	for _, f := range dir {
-		post, err := getPost(fileSystem, f.Name())
+		fileName := f.Name()
+		if !isMarkdownFile(fileName) {
+			return nil, ErrUnknownFileType
+		}
+		post, err := getPost(fileSystem, fileName)
 		if err != nil {
 			return nil, err
 		}
@@ -19,3 +31,9 @@ func NewPostsFromFS(fileSystem fs.FS) ([]Post, error) {
 	}
 	return posts, nil
 }
+
+func isMarkdownFile(fileName string) bool {
+	splitted := strings.Split(fileName, ".")
+	_, ok := markdownSuffix[splitted[len(splitted)-1:][0]]
+	return ok
+}
diff --git a/blogposts/blogposts_test.go b/blogposts/blogposts_test.go
index ca5f8f2..702aa15 100644
--- a/blogposts/blogposts_test.go
+++ b/blogposts/blogposts_test.go
@@ -56,6 +56,17 @@ World`,
 	})
 }
 
+func TestWrongFile(t *testing.T) {
+	fs := fstest.MapFS{
+		"hello world.txt": {Data: []byte("Yolo")},
+	}
+
+	_, err := NewPostsFromFS(fs)
+	if err == nil {
+		t.Errorf("should be an error but not")
+	}
+}
+
 func assertPost(t testing.TB, got Post, want Post) {
 	t.Helper()
 	if !reflect.DeepEqual(got, want) {