Replacing shower head - thread sticking out of wall is too long
It is really hard to tell from the picture. Is the pipe from the wall threaded all the way down to below the tile surface?

If the answer is no then I would be concerned that if you were to saw off 10mm of the threaded pipe coming out of the wall that you may have one or two problems.


You could be left with so few threads that it will be hard to seal the threaded joint without a leak. 
The new shower head attachment could bottom out on the threaded portion of the pipe and still not be tight to the tile surface. 


+ + Edit + +

Based on the comment left by the OP I would fully agree that the extending pipe can be cut to a shorter length. As a matter of fact, when looking closely at the provided picture it is easy to see that this pipe has already been cut from a previous install. 

When cutting off a threaded item like this there should be some post cutting dressing of the first thread using a file. The tapering down of the leading edge of the first thread and a general beveling all the way around the outside corner will greatly ease installation of the mating shower head assembly. It the sawing leaves any sharp edge or burrs on the inside corner of the pipe those should also be removed.

When cutting off this pipe to the requisite 8 to 10mm protrusion it will be best done using a hand tool such as a good quality hack saw with a new fine toothed blade. I would recommend taping some cereal box type cardboard to the tiles before starting the work in order to prevent the hack saw from rubbing on the tile and grout lines and leaving metal markings which can be hard to clean away. It may also be suggested to wear some of the thin hand forming gloves similar to "Gorilla Gloves" to avoid getting skinned up hands working in close to the wall.