It will be added in the next update. For now you can use this code:
B4X:
Sub IsDirectory(Dir As String, FileName As String) As Boolean
Dim filetype As String = File.GetAttributes(Dir, FileName).Get("NSFileType")
Return filetype = "NSFileTypeDirectory"
End Sub
Error getting attributes: Error Domain=NSCocoaErrorDomain Code=260 "The operation couldn’t be completed. (Cocoa error 260.)" UserInfo=0x170268400 {NSFilePath=/var/mobile/Containers/Data/Application/D694812A-9867-438B-8A4D-13BA5FE25CC2/Documents/B4iDir, NSUnderlyingError=0x170058720 "The operation couldn’t be completed. No such file or directory"}