Improve UX around linking and attaching images and files (attachments)

Describe the bug
Text offers different ways to add an image or file to the editor and they result in different representation of that file. This is confusing to users and we should rethink it.

On the one hand, we distinguish between:

1. attachments (being it an image or any other file) which live in the .attachments.<fileId> folder
2. links to images/files that might live anywhere within the Nextcloud instance (also possibly in .attachments.<file>

The first is rendered via ImageView.vue and only supports opening full-screen in either ImageModal.vue for images or viewer/new tab for attachments depending on whether Text is already opened in Viewer. Images are displayed directly, but other non-image attachments only show a mimetype-related icon and don't over embedded preview.

The second is a classic link to the file and can be rendered as preview via reference widgets from different apps, optionally with embedded in-place editing capabilities (e.g. for office documents or deck boards).

The resulting user experience is very confusing. E.g. users might want to upload a file to the document as real attachment, but still might want a preview view of this file.

Also related: #8162