Hi everyone,
I'm trying to open an attachment in .mpp format and my computer asks which program to use. Does anybody know?
Thanks

You need Microsoft PROJECT to open this.
There are a number of project viewers around.
project viewer - Google Search

Microsoft recommends the Steel Ray Project Viewer. It is a commercial viewer, meaning it isn't free, but it DOES have a trial period allowing you fully test it out and view your existing .mmp file(s).

Even if you only need to view this one .mmp file, they recommend this one due to the free trial giving you a better product to do your viewing in. If you are going to need the ability to view .mmp files for a long time, you CAN try a free one, or pay the small licensing fee for the recommended commercial viewer.

Steel Ray Microsoft Project Viewer (for the PC, Mac, and Linux)