Initial implementation based on libreoffice conversation

[ChristophWurst]

Feature request

As a Nextcloud user I want to view the contents of a EMF files. These might be sent via an email, so I open them with Viewer in Mail, or they are shared via Nextcloud Files and I open them with Viewer as well.

Implementation

A .emf (application/octet-stream) file can't just be opened by the browser, but it can be converted to a viewable representation. Libreoffice can convert these with libreoffice --headless --convert-to png input.emf. This Nextcloud app can add a simple route to which a EMF file can be sent and converted. Sending the file makes the feature usable for attachments in Mail and also files on DAV.

Work packages

• Handlers based on file extension, rather than MIME viewer#2008
• Create an endpoint for the conversation
• Extend viewer with a new handler but reuse the existing image handler
• Documentation
  • Requirements: libreoffice installation

[ChristophWurst]

This is done via nextcloud/viewer#2065, nextcloud/server#41395 and nextcloud/richdocuments#3303