Mount sparsebundle with a NAS WD My Cloud ex2

I use TimeMachine with a NAS. It work well and it mount a disk image when doing backup or when I enter timemachine.

When the disk image is mounted, I can access all the directories of my backups.

However, I can't mount myself the disk image from the sparsebundle. If I try to mount it, the finder shows an error after few minutes (I don't know what is the exact name of the error in english).

Is it a simple way to overcome this drawback ?

I have got a WD Mycloud ex2 and a MacBookPro with snow leopard.