The
Directory.EnumerateFiles
method can only accept one type of mask, which is what you're doing currently. You can use LINQ to compile an aggregate collection of records:
string[] extensions = { "*.txt", "*.jnl" };
DirectoryInfo directory = new DirectoryInfo(fbd.SelectedPath);
IEnumerable<string> files = extensions.SelectMany(
e => directory.EnumerateFiles(e, SearchOption.AllDirectories)
);
foreach (string file in files)
{
Console.WriteLine(file);
}