I'm having the same issue at my company. We have a group of employees with company phones who can't open attachments.
We've gone through the trouble shooting options:
1. Check for system updates
2. Remove and reinstall e-mail
3. Factory reset (tried with one phone)
4. Tried removing the cache per above.
None of these things are working. Any other suggestions? Is there another system update that is going to come out to resolve the issue?