Here is the other connection string you will want to use for .xlsx files:
"Provider=Microsoft.Ace.Oledb.12.0;Data Source=" + filepath + ";Extended Properties=" + "\"" + "Excel 12.0;" + "\"";
You already have the connection string for .xls files. Just check the extension as I said above and then apply the appropriate connection string.
Also, be sure you are using the full file path and not just the name in your connection string.