fix(files): Don't fail if dir exists

[solracsf]

Summary

In some cases, mkdir is returning false because directory already exists on disk (race condition?)
Nevertheless, we shouldn't need to throw if folder exists.

Exception":"OCP\\Files\\NotPermittedException","Message":"Could not create folder \"/appdata_ocwfs52oscxg/preview/5/e/8/b/b/c/e/3685938\

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)

[solracsf]

/backport to stable29

[solracsf]

/backport to stable28

[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.)