Add jp2 support

[mexthecat]

Could you please add jpeg2000 to the previewSupportedMimes.
Would be cool.
Thanx

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[skjnldsv]

Thanks for this, and sorry it sat so long.

Your change was right, it just had nothing behind it. This list is filtered
against the server's enabled preview providers, and nothing in Nextcloud
claimed image/jp2, so the entry got dropped again on every server. That is
presumably why it never went anywhere.

So we did the missing half. nextcloud/server#64603 adds OC\Preview\JP2,
which is merged and lands in 36, and the entry itself is now in the viewer
library as nextcloud-libraries/nextcloud-viewer#53 with your co-authorship on
the commit.

Closing here because this repository is frozen: the viewer moved to
https://github.com/nextcloud-libraries/nextcloud-viewer as @nextcloud/viewer
and master no longer takes changes, only the stable branches until they go EOL.

One caveat so you know what to expect. ImageMagick is the only backend that
reads JPEG 2000 (libgd has no support at all, and Imaginary returns 406), so
like TIFF and HEIC the provider is off until an admin adds OC\Preview\JP2 to
enabledPreviewProviders. It will not work out of the box on most servers.