Files for Notes / Blogposts?

Since Kirby3 its not possible to add files through it. But i need it for my thumbnail pictures, its only possible to use files for pages. Why? How can i reenable it?

What do you mean by:

its only possible to use files for pages

V3 doesn’t have neither a files nor a subpages section by default but you can add those back quite easily.
If you can tell us what do you plan to build we can probably point you towards the right direction

If you’re after the “old” Kirby 2 experience you can check the Page preset (first on the list here https://getkirby.com/docs/guide/blueprints/presets)

I just want the field back with the option to upload a file for a blogpost.

There was never a field that allowed uploading a file. But in any case, you will have to add a files section to upload files.

If you use a page preset (as suggested by @manuelmoreale), a section to add subpages and to upload files is automatically added to the form.

It’s now possible to add images via the textarea field and an upload function for the files field is on the roadmap for the next feature release.