Outlook Web App now includes full integration with OneDrive for Business which allows the users to share files stored in the cloud as attachments in their email. Basically there are 2 ways you can share a file with Outlook Web App and OneDrive for Business:

• When you send an attachment from your computer or device you can now automatically upload the file to your OneDrive cloud drive and send it as a link.
• You can also easily attach a file directly from your OneDrive cloud drive when sending an email in Outlook Web App.