get_all_files.gd

func get_all_files(path: String, file_ext := "", files := []):
	var dir = Directory.new()

	if dir.open(path) == OK:
		dir.list_dir_begin(true, true)

		var file_name = dir.get_next()

		while file_name != "":
			if dir.current_is_dir():
				files = get_all_files(dir.get_current_dir().plus_file(file_name), file_ext, files)
			else:
				if file_ext and file_name.get_extension() != file_ext:
					file_name = dir.get_next()
					continue

				files.append(file_name)

			file_name = dir.get_next()
	else:
		print("An error occurred when trying to access %s." % path)

	return files

If you want the full path of the file instead of just the name, change:

Line 17

files.append(file_name)

for

files.append(dir.get_current_dir().plus_file(file_name))

Upgraded version for godot 4.x

func get_all_files(path: String, file_ext := "", files := []):
    var dir = DirAccess.open(path)

    if DirAccess.get_open_error() == OK:
        dir.list_dir_begin()

        var file_name = dir.get_next()

        while file_name != "":
            if dir.current_is_dir():
                files = get_all_files(dir.get_current_dir() +"/"+ file_name, file_ext, files)
            else:
                if file_ext and file_name.get_extension() != file_ext:
                    file_name = dir.get_next()
                    continue
                
                files.append(dir.get_current_dir() +"/"+ file_name)

            file_name = dir.get_next()
    else:
        print("An error occurred when trying to access %s." % path)

    return files

still works (godot4.2) + small improvement

## returns list of files at given path recursively
## [br]taken from - https://gist.github.com/hiulit/772b8784436898fd7f942750ad99e33e
static func get_all_files(path: String, file_ext := "", files : Array[String] = []) -> Array[String]:
	var dir : = DirAccess.open(path)
	if file_ext.begins_with("."): # get rid of starting dot if we used, for example ".tscn" instead of "tscn"
		file_ext = file_ext.substr(1,file_ext.length()-1)
	
	if DirAccess.get_open_error() == OK:
		dir.list_dir_begin()

		var file_name = dir.get_next()

		while file_name != "":
			if dir.current_is_dir():
				# recursion
				files = get_all_files(dir.get_current_dir() +"/"+ file_name, file_ext, files)
			else:
				if file_ext and file_name.get_extension() != file_ext:
					file_name = dir.get_next()
					continue
				
				files.append(dir.get_current_dir() +"/"+ file_name)

			file_name = dir.get_next()
	else:
		print("[get_all_files()] An error occurred when trying to access %s." % path)
	return files

use like this:

var level_scenes := FileUtils.get_all_files(levels_folder_path, ".tscn")