Code error message

"line 8, Argument 1: cannot convert 
from'System.Collections.Generic.List<string>' to 'string'

List<string> strs1 = new List<string>();
string[] strArrays = Directory.GetFiles(str1);
for (int j = 0; j < (int)strArrays.Length; j++)
{
    strs1.Add(Path.GetFileName(strArrays[j]));
}
richTextBox1.AppendText(string.Concat("New files in folder", str1, " : "));
richTextBox1.AppendText(strs1);