Add filename in the preview docs

[MorrisJobke]

cc @jancborchardt@tobiasKaminsky@nickvergessen

For https://github.com/nextcloud/activity/pull/576/files#r630185430

[MorrisJobke]

I just wonder why this is added to the preview and not to the element itself 🤔

[MorrisJobke]

ref #576

[MorrisJobke]

@nickvergessen Tobias wondered yesterday if this should be seen as breaking change? There are JSON parsers out there that also break if new elements are added.

[nickvergessen]

nickvergessen Tobias wondered yesterday if this should be seen as breaking change? There are JSON parsers out there that also break if new elements are added.

Adding additional items is per semver not a breaking change, so it should be fine. No need to duplicate the API unnecessarily.
Also any app could create a new object type and the clients should gracefully survive this.

I just wonder why this is added to the preview and not to the element itself 🤔

Because the preview object is disconnected from the files object (not every file can has a preview) and it was just easier this way to inject the data on the level it is needed and available instead of starting to map values later on.

[tobiasKaminsky]

Adding additional items is per semver not a breaking change, so it should be fine. No need to duplicate the API unnecessarily.
Also any app could create a new object type and the clients should gracefully survive this.

ok, did not knew this. I already checked with other clients to see that everything is working.