fix: adjust file picker for background image to allow folder navigation

[cristianscheid]

• Resolves: [Bug]: Can't put custom wallpaper / background from personal space #58645

Summary

Currently, the file picker to change user background (under settings/user/theming) does not allow to navigate folders when searching for a file. Instead, it only shows files from root directory on "All files" tab.

This happens because of setFilter((node) => node.mime.startsWith('image/')), which causes folders to be filtered out from results. This PR changes this line to use setMimeTypeFilter(['image/*']), which allows to navigate folders while mantaining the filter to only allow image selection.

Screenshots

Before	After

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

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

[susnux]

If possible it would be really great to have a small PlayWright test for this