Checklist

• I am using an up-to-date version.
• I have read the documentation.
• I have searched existing issues.

Description

Krita (.kra, .krz) and Open Raster (.ora) files are currently not supported for thumbnail generation.

Solution

Support thumbnail generation for these file types.
They are all standard zip files.
.kra and .ora usually contain a full resolution "mergedimage.png" at the top level. .ora files adhering to an older specification might not include this.

.kra and .krz should have a scaled down "preview.png" at the top level. It appears to be scaled so that the longer side is 256 pixels.
.ora has this file in "Thumbnails/thumbnail.png"

Test.zip contains the same image in each format. They are packed into a zip because github wouldn't let me attach them with their original endings.

Alternatives

No response