test(theming): cover background file picker mime filtering

[mvanhorn]

• Resolves: [Bug]: can not select custom background #62105

Summary

This is a regression introduced by commit 15c336f (2026-07-01, "adjust file picker for background image to allow folder navigation"), which replaced the working image filter .setFilter((node) => node.mime.startsWith('image/')) with .setMimeTypeFilter(['image/*']) in the pickFile() function. The intent of that change was correct (a bare setFilter predicate hides folders too, so users could not navigate into subfolders), but the ['image/*'] mimetype-filter path yields an empty browse list in practice, while the other image picker in the codebase (apps/settings/src/components/PersonalInfo/AvatarSection.vue) uses explicit types ['image/png', 'image/jpeg'] rather than the glob. Fix pickFile() by restoring a working image filter that also keeps directories visible for navigation: replace .setMimeTypeFilter(['image/*']) with a folder-aware predicate .setFilter((node) => node.type === 'folder' || node.mime?.startsWith('image/')) (this mirrors the FilePicker's own internal rule of always showing folders plus matching files, and restores the pre-regression image display without reintroducing the folder-navigation problem). This is the delta versus the 2026-07-01 change: it keeps folder navigation but stops the browse list from coming back empty. The fix is self-contained in UserSectionBackground.vue; its only consumer, apps/theming/src/views/UserTheming.vue, needs no change. Add a focused unit test that mocks @nextcloud/dialogs (to capture the filter callback passed to the builder) and @nextcloud/initial-state (the component reads several loadState values at module load), then asserts the predicate accepts a folder node and an image/png node and rejects a text/plain node.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests are included
• Screenshots before/after for front-end changes
• Documentation has been updated or is not required
• Backports requested where applicable
• Labels added where applicable
• Milestone added for target branch/version

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[mvanhorn]

Done in 3b6c67f — added .setCanPick so only image files are selectable while .setFilter still lets you navigate folders, and moved the spec out of __tests__/ next to the source. Thanks!

[susnux]

Thank you :)

Please rebase onto current master branch. Also you need to signoff both commits (to pass the DCO check).
Moreover as this is AI generated / assisted please add the matching line for disclosure to your assisted commit according to our AI policy:
https://github.com/nextcloud/.github/blob/master/AI_POLICY.md#disclosure

[mvanhorn]

Rebased onto current master with signoff. While rebasing I found the picker fix itself already landed via the setMimeTypeFilter change, so I slimmed this down to what master is still missing: regression coverage for the picker (image mime filter applied, folders navigable). Can retitle if you'd like.

[github-actions]

Hello there,
Thank you so much for taking the time and effort to create a pull request to our Nextcloud project.

We hope that the review process is going smooth and is helpful for you. We want to ensure your pull request is reviewed to your satisfaction. If you have a moment, our community management team would very much appreciate your feedback on your experience with this PR review process.

Your feedback is valuable to us as we continuously strive to improve our community developer experience. Please take a moment to complete our short survey by clicking on the following link: https://cloud.nextcloud.com/apps/forms/s/i9Ago4EQRZ7TWxjfmeEpPkf6

Thank you for contributing to Nextcloud and we hope to hear from you soon!

(If you believe you should not receive this message, you can add yourself to the blocklist.)