Hello,
When there are many files, and I wish to download one only, are automatically selected also the files following the only one I wish.

see example:
http://i891.photobucket.com/albums/ac115/eridano_bucket/Tixati%20bug_zpscmdt1k7m.jpg

send that torrent to the devs. support at tixati dot com

There is no "middle file" logic.
The auto- file select assumes LARGEST is the wanted one and you can DESELECT anything 1-99% of that largest file size. Your specific file, this does not work because you do NOT want the LARGEST file.
It is possible those last three files are in the same piece and as such will be downloaded no matter if you only select file #2.