What you are missing here is that these files this guy is sharing are essentially just descriptions of shapes and therefore typically would be considered speech. The files then let you make nuclear weapons (though really poor quality ones). He is sharing information though, not nuclear weapons, which is why this has been transmuted from a second amendment issue to a first amendment one.
Arms=small arms and nuclear arms. Free speech famously has limits (falsely yelling "fire" in a crowded theater) so where do we draw the line here?