[release-2.19] fix: skip thumbnail generation for GIF images

[halo-dev-bot]

This is an automated cherry-pick of #6597

/assign guqing

修复 GIF 缩略图生成只会保留第一帧的问题

[f2c-ci-robot[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ruibaby

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/halo-dev/halo/blob/release-2.19/OWNERS)~~ [ruibaby] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.

Please upload report for BASE (release-2.19@a45ebb0). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...n/halo/app/core/attachment/ThumbnailGenerator.java	14.28%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-2.19 #6609 +/- ## =============================================== Coverage ? 58.05% Complexity ? 3917 =============================================== Files ? 670 Lines ? 22966 Branches ? 1575 =============================================== Hits ? 13334 Misses ? 9015 Partials ? 617 ```

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