How I can I block mail with certain attachments or certain words at server level?

A. You can't do this in Exchange server. You would have to use a third party application such as EMail Essentials for Exchange or Mimesweeper.

To setup a the content checking feature in Mail Essentials;

Start-up the Mail essentials configuration
Go to the content checking tab
Now you can enter:
a. types of attachments that must be blocked
b. mails with particular words/phrases in the subject that must be blocked
c. mails with particular words/phrases in the body that must be blocked
Click here to view image
Any mail that is blocked is quarantined in the moderator client.

Administrators/supervisors can then check the mail and either approve or reject it.