Fix: Allow preview and edit of native Excalidraw and TLDraw files

[hackeresq]

Excalidraw files are currently non-portable because seafile is using the .exdraw file extension. The same thing is true with TLDraw files.

According to the Excalidraw docs, the file extension should be .excalidraw:

Similarly, TLDraw files typically use the .tldr extension.

This PR enables interoperability and use of both the native extensions and seafile-specific extensions without any breaking changes.

[hackeresq]

Happy to break this into two separate PRs if we don't want to do both Excalidraw and TLdraw in the same. Hoping we can get this into the next release