Add support for .excalidraw files to add a custom mimetype

[juliushaertl]

Currently Nextcloud is not aware of .excalidraw files as a filetype. We should add support for it. Currently there is no API to do that as an app, so it would need to be done in server

nextcloud/server#35176 can be seen as a reference.

Once done there is a small frontend change required to adapt the mimetype in the viewer app integration here to it https://github.com/nextcloud-gmbh/whiteboard/blob/main/src/main.tsx#L67

[grnd-alt]

PR by @hweihwang for reference: nextcloud/server#45353