Introduction
Long press the item that you wish to send, and then tap Share. Tap which mail app you wish to use.
Instructions
1) Long press the file you wish to attach to the email message. Then, tap the Share icon at the top.
TIP: You can tap more items to select them before tapping the "Share" icon.
2) Tap the mail app that you want to use for sending the file(s).
Outcome
You have attached your file from OneDrive for Business to your email message.