Migrate Deck attachments to File attachments

Background

Deck 1.3.0 introduced File attachments. New attachments are uploaded to the user directory as files. This brings up many advantages compared to the previous custom handling of attachments. One benefit is an appropriate thumbnail generation (for example for cover images).

Problem

However it also introduced a technical debt: The Deck server app as well as all clients need to support both types of attachments. Especially when displaying cover images, the user requires to download the whole image regardless of its size to display it. This increases the maintenance efforts for the developers and is a bad user experience due to large mobile traffic and load times.

Solution

In https://github.com/nextcloud/deck/pull/2638#issuecomment-740644439 we discussed this topic and a possible solution was to migrate existing Deck attachments to File attachments. As this can be a long running job, @juliushaertl decided to not automatically perform the migration when updating to Deck 1.3.0. This issue will be solved when

• an occ command for a manual migration is offered
• administrators are notified about the availability of the occ command and are prompted that it is recommended to execute it
• the next Deck major release will perform missing migrations automatically

No files or tests are named. Start by tracing the existing Deck attachment handling and the occ command entry point, then review the linked discussion in PR 2638. Done means a manual migration command exists, administrators are informed and prompted, and the next major release can perform remaining migrations automatically.