Resource types

I have uploaded some .c files as resources. Well, when I download these files from my home computer I get files with .c extension, correctly.
But when my students try to download those files with their PC, they get files with the extension .c.txt.
For example: the uploaded file was manyfork.c and the downloaded file is manyfork.c.txt.
I tried from the computer on the teacher's desk, at school, and I confirm that on all the computers in the school the files are downloaded as manyfork.c.txt.
I do not think this is a Moodle server problem because the files are downloaded correctly from my home computer.
So, what could be the reason for this behaviour?
I use Moodle 3.5 on a Moodlecloud hosting.

Thank you so much, Luigi.