File incompatibility with excalidraw.com

Steps to reproduce

• Create a new drawing on excalidraw.com (or another excalidraw-based application)
• Upload the .excalidraw file to nextcloud
• Open the file and save it
• Download the changed file
• Attempt to open the file via excalidraw.com

Why is this a problem

It sometimes makes sense to use Nextcloud to save excalidraw files wile editing them elsewhere (example obsidian, excalidraw.com, etc.)

Opening these files on the nextcloud web-ui, whether to edit them or by mistake, "corrupts" the file so it can no longer be opened elsewhere. This makes it impossible to use both to edit files depending on the situation, and makes it easy for users to break their drawings in a way that requires tinkering to fix.

Workaround and possible fix

A downloaded .excalidraw file can easily be fixed by prepending the following JSON values:

{
  "type": "excalidraw",
  "version": 2,
  "source": "https://<my-nextcloud-domain>",
}

Afterwards, both whiteboard and other tools can open the file again, unless it is saved by whiteboard and broken again.

Reproduce the workflow using an .excalidraw file from excalidraw.com: upload it to Nextcloud, open and save it in the web UI, then download it. Trace how the file is serialized when saved and compare it with the format expected by excalidraw.com. Done means a file saved through Nextcloud opens again in excalidraw.com and other Excalidraw-based applications.