http://filext.com - THE place to see what the extention is.
If the eps file isa attahced, here's what you can do: Save the attachment to your desktop (if you're running Windows). Then hold down the LEFT shift key, and RIGHT click on the file. This will bring up a menu, that'll have "Open With" on it..click on that, see if there's a default program. If not, click CHoose program, select a graphics viewer that your system has (photoshop, Windows Picture Viewer, whichever). You can also click on the "always use this program to open files of this type" which'll set set the program you choose to always use that program when you double click on an EPS file.