In Gmail, click Compose and select Cloud Services in the LiveHive toolbar. This allows you to attach a file located in a cloud service.
In the popup, select the cloud storage service that you use. The options are Box, DropBox, Egnyte, and OneDrive.
The next popup prompts you to sign in to the cloud storage account. Once you've signed in once, you will not be prompted to sign in in the future. See below for an example of the sign in page for OneDrive.
Once signed in, you'll see all of the files in your account.
Select the file you want to attach. Once you click on the name of the file, it will be added to the email as a tracked attachment.