FIX: Can't open excel documents
I know the OP is old, but I've been searching for a fix for this problem for a while.
I found lots of suggestions on line (delete and add the account; clear data and cache from eMail app; reboot phone

- unfortunately nothing worked for me. I also found suggestions to download apps that would open the attachments... but, I didn't want to do that - and, I had been able to open attachments, so I realy wanted to know what had changed.
I figured it out, and thought I'd post a reply. If it's too late to help the original poster, maybe it will help someone else searching in the future.
The Samsung S3 & the Galaxy Tab 2 (and maybe other Samsung models as well), come preloaded with an app to view Microsoft files (Excel, Word, etc), called Polaris Viewer and/or Polaris Office (I don't think you can get these apps new from the PlayStore). Anyways - all I needed to do was go to my
SETTINGS, then
MANAGE APPS, view
ALL, and near the bottom of the list I found Polaris - and it showed disabled. Just enable this app.
Worked like a charm for me! I hope this helps someone else.