fix(files): do not even try to fetch a preview if has-preview is false

[skjnldsv]

Prevent one additional request in case the backend said there was no preview.
This property does NOT mean the preview was not generated, but it actually means we CANNOT generate it (disabled or no preview available)

Good suggestion by @icewind1991 !

[skjnldsv]

/backport to stable31

[skjnldsv]

/backport to stable30

[susnux]

I do not really expect performance improvements as mimeFallback on the preview endpoint should also check this so so we already would get the mime type icon.

Also one thing to consider is that doing this will stop the browser from caching the mime icon as the preview, so other apps might need to do this as well otherwise we do not save anything 😔

[skjnldsv]

This created issues, for some reason I got many img loading errors after this PR.
I refactored the handling a bit and unified the way we set and reset the preview to avoid prop update race conditions/conflicts

[skjnldsv]

I do not really expect performance improvements as mimeFallback on the preview endpoint should also check this so so we already would get the mime type icon.

This removes one request :)

[skjnldsv]

Now cancelling doesn't work 🛑

fixed