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Gmail lets you view and save attachments directly to Google Drive from within your mail message. 

Attachments are visible as previews at the bottom of a message. Clicking on the preview brings up a fullscreen view of the file, and you can navigate through several attachments the same way you flip through a photo slideshow.

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Click on the Google Drive icon that appears when you hover over an attachment and choose where to send your attachment.

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This is a great feature for those using the GMail app on an iPad or iPhone.