I’d like having the chance to share attachments, maybe text or pdf files, but mostly media (images, audio, video), so receivers in the chat could read/view/watch/listen at them, but they wouldn’t be able to download them or share again.
I believe that if I share some media/attachments, the receivers in the chat can download them and share again with other people.
Telegram has a feature to “disable sharing” in a chat.
Obligatory disclaimer: there is no fool-proof way to actually prevent such sharing, but it’s of course possible to make it good enough for most people.
not to sound rude but i hope delta chat won’t implement this because this would just open to forks and fragmentation of yet another good open project 
If they are able to decrypt a message, they are able to do whatever they want with that message, it’s the crux of our digital lifes but it’s here to stay (well, unless the client becomes as pervasive as checking the whole os to be sure that nothing is gonna take that packet etc etc but it would be way a lot of work and much more prone to dystopic scenarios)
If you don’t trust the person you are messagging with, there is not much to do. Delta help in this anyway, letting you make as many profiles as you want so you can at least have some plausible deniability 
Signal has “view once” media which is an ok-ish way to say “check this but please don’t store or forward it anywhere”.
If you don’t trust the receivers not to screenshot the message and not take a photo or video, this is not a solution, better not send the messages to them.
We have “disappearing messages” already, but it is a way to say that you don’t want the message to be stored forever to contacts you trust, especially in a group where someone may not pay attention that you asked not to store messages and not care to delete them. E.g. you send a photo of your documents to someone in a family because they need it now for some paperwork but don’t want the documents to be stored on all your and family member devices forever. Disappearing messages are fine to ensure nobody forgets to delete the document photo week later.
“View once” media can also be ok for similar situation, e.g. if you want to send a photo of how you look like to some people you want to meet on the street who you have talked to only online before, but don’t want them to have a photo of you stored pretty much forever way past it is useful. Or you want to send a voice message because you cannot type right now, but don’t want your voice recording to be stored forever.
But I also see no way Delta Chat builds in “DRM”, if you see contacts as adversaries just like media companies who use DRM view their customers, better not send them messages but show what you want to show in person from your screen and make sure they don’t take photos.
Thank your for all the answers. Both the disable sharing and particularly the “View once“ seem good solutions, even if they are partial solutions.
A couple of comments.
It was edited indeed, by @ian
I have reverted the edit.
DRM can stand for either digital rights management or digital restriction management. It refers to technologies through which the author or beneficiary of a given work can enforce usage restrictions upon recipients (i.e., such as by restricting whether they are authorized to project it, print it or save it).
I prefer to not touch the part of the post title that you type in and instead add remarks after it so that I (and others) can better understand the topic in case its wording could be improved (essential for both having an overview and to keep later replies on-topic). Otherwise, I would probably rename this topic to something like:
When rendering such content on screen, it is also usually marked as protected at the OS level to disallow taking screenshots of it. A funny story about this:
Thanks for the explanation. You may rename the discussion as you like (I do not know if I can edit the title). I’ve just commented as I didn’t know the meaning of DRM.
Thank you. I’m usually cautious not to change the existing text of others because they may then not be able to recognize it as their own post later. Sorry that the note turned out to be ambiguous.
This is an obscure and not very intuitive feature of Discourse that took me a while to find out. To rename a topic or adjust its tags, you need to first scroll to the very top of the page and then click on the title text (it won’t work if you scroll down just a bit). A little pencil icon would also appear on the right once you hover over the title with a mouse. Then you need to find the small checkmark on the left to save the edit.