iOS finally provides a simple way to attach files from your iCloud Drive when reply or sending a mail on iPhone. To add attachment to email on iPhone, iPad and iPod Touch devices, do as follows:
1. Open the Mail application on your iPhone (must be running iOS 9 or later)
2. Start composing a new mail or reply to a mail
3. Tap and hold you finger on a place in the message body area
4. An action box will appear and includes several options like: copy, paste or select
5. Swipe all the way to the left, skip the Insert Photo or Video option, until you get to the Add Attachment option (You may also use the arrow-like keys to move between actions)
6. Tap the Add Attachment button and your iCloud Drive will be opened
7. Select the content you want to share via Email and you are all DONE.