Antonio,
It appears that somewhere between 16.03 and 16.08 drag and drop functionality has been broken again in 64 bit apps. I know it was working, I think last in 16.03. I found the issue regression testing my own code, but the error is reproducible if you build testdrop.prg as a 64 bit app. Anything dropped on testdrop built as a 64 bit app will hit the following zero files condition and popup the message box. 32 bit seems to work fine, only 64 bit is effected.
46 if( wFiles == 0 )
47 MessageBox( 0, "zero files", "ok", 0 );
Unfortunately I don't have a copy of the 16.03 64 bit files available, but I confirmed this is broken in all builds from 16.04 through 16.08. I do not have a workaround.
Robb