Attachment not shown if Viewer app not installed

[odzhychko]

Steps to reproduce

1. Have a setup without Viewer app
2. Follow instructions from Attachment disappears when sending a message with an inline image and a regular attachment #13452

Expected behavior

If Viewer is not installed, do not offer preview.

Actual behavior

Visual elements for the attachments do not render at all.

Also get error as posted in #13452 (comment)

Mail app version

No response

Nextcloud version

No response

Mailserver or service

No response

Operating system

No response

PHP engine version

None

Nextcloud memory caching

No response

Web server

None

Database

None

Additional info

https://github.com/nextcloud/mail/blob/v5.10.11/src/mixins/AttachementMixin.js assumes OCA.Viewer exists.
We should check if it exists and fallback gracefully, if it doesn't

[ChristophWurst]

cc @hamza221

[hamza221]

The hardening doesn't hurt, But don't we assume that viewer exists ?

[ChristophWurst]

The app is shipped and default enabled but it's not always enabled. This needs a check.