Transparent PNGs don't cast shadows (in BI) Blender 2.77 - Blender Internal

Hello, I'm trying to make a simple animation about files entering an envelope. I have some troubles with "Import Images as Planes", they don't cast transparent shadows, despite I checked "Raytrace" (or Z-transparency) on them, and "Receive transparent shadows" on the envelope's material.

Envelope material settings, PNG settings, and the result:



Here's the full project:

https://drive.google.com/open?id=0B_9DH8lxMw2bSE9hbWNPXy1DWm8

The example is frame # 397

After days and days, I still can't understand where I go wrong. Please help...
Select the "jpg" object and uncheck the "cast" option in the shadow tab.



If you want your object to cast a ray traced shadow you must add a ray tracing light (i.e. a sun lamp) and enable her ray trace option.
In your animation you will also have to move the object toward the camera to separate the plane of the object from the plane of the cast shadow.