fix(preview): Validate preview MIME type

[solracsf]

• Resolves: [Bug]: preview generation crashes on image files when generated preview has null MIME type #58784

Summary

When generating previews, malformed dimensions can produce a preview image with a null MIME type.

This validates the preview object and its MIME type after resize/crop operations and before creating the DB record. If the preview is invalid or has a null MIME type an InvalidArgumentException is thrown and the file is skipped instead of crashing.

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

[CarlSchwan]

/backport to stable33

[backportbot]

The backport to stable33 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable33
git pull origin stable33

# Create the new backport branch
git checkout -b backport/58793/stable33

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 304c9fbc

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/58793/stable33

Error: Failed to push branch backport/58793/stable33: remote: Invalid username or token. Password authentication is not supported for Git operations.
fatal: Authentication failed for 'https://github.com/nextcloud/server.git/'

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[solracsf]

/backport to stable33

[solracsf]

/backport to stable32

[backportbot]

The backport to stable32 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable32
git pull origin stable32

# Create the new backport branch
git checkout -b backport/58793/stable32

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts, resolve them
git cherry-pick 304c9fbc

# Push the cherry pick commit to the remote repository and open a pull request
git push origin backport/58793/stable32

Error: Failed to check for changes with origin/stable32: No changes found in backport branch

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.