1) Click on the Settings gear button.
2) Choose Email Sync from the menu on the left.
3) Check the box under Chatter Attachments that allows you to save email attachments to your Chatter Feed.
Note - Your Salesforce admin will need to enable Chatter feeds on the Task object and any objects to which you want to save attachments into the Chatter feed (e.g. Leads, Contacts, Accounts, Opportunities, Cases, etc.). See the Salesforce help page for more information: