Many Mail server has a 10MB limit. To be able to share as the ad on delta.chat says:
Unbegrenzte Chats, Bilder, Videos, Sprachnachrichten und mehr. Multi-Client-fähig.
allow to add a Nextcloud login to delta.chat.
If all attachment together are grater then 10MB, ZIP the files and put it into a specific Nextcloud directory and Link the files into the e-Mail.
Does the nextcloud app already allow to share weblinks to uploaded files?
Then you may already be able to manually do something like this. Upload files (or have the nextcloud app upload them) and share the link with deltachat.
Sure, that’s possible. But i don’t think any normal user would ever do such a detour.
Ok, so it may be needed that nextcloud provides android “intents” for other apps to support sharing files through nextcloud, e.g. like making a file selection as if selecting photos from the gallery but returning weblinks instead of file pointers.
Not sure. As far I know Nextcloud itself has an api which also the nextcloud-client use for his connection.
https://docs.nextcloud.com/server/14/developer_manual/client_apis/
That’s the web-api for the nextcloud server I think, but good integration into the android user interface would require “intends” I think. It would also avoid to have to deal with nextcloud accounts in deltachat.
For example, when one attaches a file to a deltachat message, does the selection already contain an icon for nextcloud? These are provided by “intents”.
Sorry that’s above my knowledge. I only have some bash and powershell knowledge.
I basically thought about:
If user press “Send Message” check total filesize. If that’s more then 10MB, zip them and send it to Nextcloud server, ask for a share link and if you receive one, replace the attachment with a link.
Something like that.
It should be very easy to add that for ownCloud as well 
Well, yes, the own/nextcloud app or any other app could provide a way to “share link(s) to file(s)” -> and then selecting deltachat as the messenger to use.
But a way in deltachat to “insert (own/nexcloud) links” to the message (the files may are already be on the server) instead of adding attachments to the message, may make this much easier to use.
EDIT: Oh, only now I noticed you meant owncloud vs. nextcloud. Sure, both (any compatible intent) would be supported.
If that’s more then 10MB, zip them and send it to Nextcloud server, ask for a share link and if you receive one, replace the attachment with a link.
I think the basics should pretty much be covered by the nextcloud app plus nextcloud server, even the zipping up of multiple files for downloadin. It would make sense for the nextcloud app to make a “share file as nextcloud link” functionality available to other apps (intents). Nextcloud users are likely using the app and may have uploaded the file to share alrady. Can you confirm, or also ask for this on their part?
Once, the basics are available as “intent” functionality, deltachat may then use them to reduce email traffic or avoiding max. message size errors.
I may have Calendar and Contacts integrated in my Phone but I do not have the Nextcloud/Owncloud app installed.
To be able to use it without the requirement to have an extra app installed I would prefer a solution without it.
Frankly, I think specific nextcloud supporting code isn’t ever going to happen, because it makes no sense to rewrite and maintain nextcloud functionality in deltachat when all the user has to do (and is likely to do or already have done anyway) is installing his cloud-, webserver-, or any document-management app, that provides a file sharing intent that returns a weblink.
Thunderbird has similar functionality: https://support.mozilla.org/en-US/kb/filelink-large-attachments
Files-wise, NC has WebDAV support, which might be enough to support in a chat app.
However, now there are very interesting developments in NC Talk, integrating most variants such as Audio/screen sharing/WebRTC, chat with attachments/files support.
So it might be interesting to explore the potential of integrating with NC Talk (Android App + NC internal desktop app).
Yeah, I like the way Thunderbird did that. You can choose any “File Link” plug-in for whatever cloud file storage site you want to use. Maybe a “WebDav File Server Instance” input field in the settings would be best, similar to how Delta Chat has a “Video Chat instance” field (except it would probably need authentication settings there, too).
Another possibility might be to use Jirafeau file sharing instances. Jérôme Jutteau / Jirafeau · GitLab That’s an open-source server project similar to how WeTransfer or SendGB works where you upload a file and it gives you a link. There’s some API info here: Jirafeau, your web file repository
May a bit big, but you could add addon support in Delta-chat. May at least a subset of functions for addons.