Add attachment support for nextcloud-text

[pbirrer]

This PR integrates the new attachment support from nextcloud/text into nextcloud-drawio.

Changes

• Attachment support:
  • Adapted nextcloud-drawio to work with the new Text editor attachment feature
  • Enables inserting .drawio files directly from the Text editor as attachments.
• File previews:
  • Added a preview image that will be displayed inside Nextcloud Text when referencing a draw.io file.
• Mimetype registration:
  • Updated the way draw.io files register their mimetype to be available to other nextcloud apps via template manager.

[davidjgraph]

Please confirm this is manually written code, not AI generated.

[pbirrer]

Hey @davidjgraph, yes, this is indeed manually written code and may need some refinement 😄