fix: custom background file picker shows an empty file dialog

[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
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AI (if applicable)

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