diff --git a/tree.go b/tree.go
index 6d94874..6e03c12 100644
--- a/tree.go
+++ b/tree.go
@@ -24,10 +24,10 @@ type Config struct {
 	AttachmentDirName string
 	CustomMeta        map[string]func() string
 	Excludes          []string
-	Apps              map[string]appCallback
+	Apps              map[string]AppCallback
 }
 
-type appCallback func(dir string, content string, info os.FileInfo) (string, error)
+type AppCallback func(dir string, content string, info os.FileInfo) (string, error)
 
 type File struct {
 	Id       string
@@ -150,7 +150,7 @@ func NewConfig(
 	attachmentDirName string,
 	customMeta map[string]func() string,
 	excludes []string,
-	apps map[string]appCallback,
+	apps map[string]AppCallback,
 ) Config {
 	return Config{
 		ReadableFormats:   readableFormats,
@@ -262,7 +262,7 @@ func moveAttachments(dir string, config Config) (map[string]string, error) {
 	return attachmentRegistry, err
 }
 
-func applyApp(app appCallback, dir string, config Config) error {
+func applyApp(app AppCallback, dir string, config Config) error {
 	err := filepath.Walk(dir, func(fullPath string, info os.FileInfo, e error) error {
 		if e != nil {
 			return e