Apple has made it easier to save and upload attachments to and from the iCloud Drive app on any iOS device. To upload an attachment from iCloud Drive using the Mail app, just press and hold the body of your email draft and select the "Add Attachment" option from the pop up bar. This will open the iCloud Drive app and allow you to attach anything stored on your iCloud.
iOS 9 has made it easy to save email attachments to your iCloud Drive (and attach items from your iCloud Drive to outgoing emails), which allows you to access them from all of your devices. To save an attachment to your iCloud Drive, simply long press on the attachment until the Share options come up, then tap on the cloud icon labeled "Save Attachment." An iCloud Drive window will pop up, tap "Save to this location" at the bottom. Now you can access the downloaded files on your iPad via the iCloud Drive app.
If you use iCloud Drive, the easiest way to access it on your iPad is with the app, which is a new addition in iOS 9. It is hidden by default, so you will have to activate it. To do so, head to Settings -> iCloud and tap on iCloud Drive. Toggle iCloud Drive on if it isn't already and then toggle on "Show on Home Screen." Now you will have an icon on your Home Screen that you can tap on to access your iCloud Drive.