There are multiple ideas of solving this issue/s:
First of all for medium sized message that can still be sent via email (attachment size under the limits of most providers.
The problem here is that for most people on mobile there is still a data limit on how much data they can transfer in a month, so the current behavior of downloading everything immediately isn’t ideal - see related topic for more details on this: Addition of automatic media download feature
Then for really large files the idea is to use another transport than email:
- simple fileservers, could be configurable like videochat server currently is and also providers could provide this as extra feature to their users in the future, there is already a prototype pr exploring that idea, but its inactive right now [WIP] Upload and download large attachements through HTTP by Frando · Pull Request #1612 · deltachat/deltachat-core-rust
in essence it would be a specification that declares an interface where files can be uploaded and then returning a link from which they can be downloaded again. which is just an automated mechanism of sending a link to the file. - using a p2p transport (device to device) but that has the disadvantage that both devices need to be online at the same time for the duration of the transfer, also special software if not using deltachat is required to download the files. (unless all other mail clients implement that spec too)
See also an existing thread on this feature Large files support