That's not a file type you link to, that's a file. I don't think anybody will download it to have a look.
It's called ATT00001 without any filetype (extension). https://www.google.nl/search?q=ATT00001 makes me thinks it's done by MS Exchange. So better ask your postmaster (Exchange Manager) or the sender what it is.
Guys how can i open this type of file
type of file