There is very often a problem with sending emails which have large attachments such as a set of complex drawings, or a model.
ISP's do not like having their systems slowed down by having to handle large attachments, and very often having determined that the size exceeds their limits, and simply cancels the transfer - often by deleting the attachment[s] to the email.
There is also a risk that they become corrupted during the process of being transmitted and stored several times.
The solution is simple: the email which might have had such large attachments is structured to include a link to a web server on to which the attachments have been uploaded.
This is achieved by using FTP, which is one of the companion protocols to HTTP and its secure version HTTPS. Please see the Wikipedia definition for more information.
To the user who is used to clicking a link to open an attachment to an email there is literally no difference - except of course that the link is to a web server as opposed to the temporary directory where the email being viewed has been stored.
From an administrative point of view, once an acknowledgement of receipt has been received by the sender. Please see the TDOC Manual: select Technical Documents and then Issue Documents, and see the "Remove FTP Uploads" section.