Check Extension

Sub CheckExtn()

Dim MinExtensionX
Dim Arr() As Variant
Dim lngLoc As Variant

'Retrieve extension of file
MinExtensionX = Mid(MyFile.Name, InStrRev(MyFile.Name, ".") + 1)

Arr = Array("xls", "xlsx") 'define which extensions you want to allow

On Error Resume Next

lngLoc = Application.WorksheetFunction.Match(MinExtensionX, Arr(), 0)

If Not IsEmpty(lngLoc) Then '

'DO STUFF if it's an xls/xlsx file. Otherwise, chose next file in folder

End If