I have never seen this behavior. It is definitely not the naming scheme used to resume incomplete file downloads (look in your incomplete directory for examples).

At first, I though the number looked like a hex value, but it's only 5 characters long.

Other limitations are file name length. If your file name is over 32 characters long (on Mac), then the file name will be shortened or the file will not work properly. Obviously, this cannot be the case here.
:-)