after u touck the attachment in the email it should download (little circle with green arrow pointing down) in the notification area. pull down the notification screen and touch the file. It should then ask u which app u want to open with.
I'm running the leaked version of froyo and I haven't been able to download my gmail attachments since I installed the leak , it just keeps on saying fetching document too. I think its a bug that is probably fixed on the official ota version.