In order to save time and space/resource to store,upload and download a big file as attachment,it is recommended to use a public cloud storage such as Google Drive,Dropbox or Sky OneDrive.Details steps as below:
1) Please log into user's webmail interface
2) Navigate to Setting -> My Settings -> Connected Services and click 'New' as below,then choose the preferred storage platform:


3) Once choose the apps,please enter your access details .In this guideline,we choose Dropbox




Then you can proceed to use your Dropbox's content to share with other as below:

Now select the specific files from your Dropbox content -> Click OK

Those selected files will appear on your message to be send to recipient.

Done.