How can I convert a .dat to a .zip on a Mac?

Hi -

I friend wanted me to update his company's website which runs on a MS Exchange Server/Sharepoint set up. He zipped all the html, jpeg, and assorted text files and attached that zip file to an email. When I received it on my Mac it had been converted to a ".dat" file. He tells me that this is the result of sending a zipped file over the MS Exchange Server mail system to a Mac. (Whatever happened to cross-platform?)

Is there a Mac utility I can use to convert this dat file back to a zip file that I then can open on my Mac?

TIA

Josh