fix: skip thumbnail generation for GIF images

[guqing]

What type of PR is this?

/kind bug
/area core
/milestone 2.20.x

What this PR does / why we need it:

修复 GIF 缩略图生成只会保留第一帧的问题

Which issue(s) this PR fixes:

Fixes #6596

Does this PR introduce a user-facing change?

修复 GIF 缩略图生成只会保留第一帧的问题

[guqing]

/cherry-pick release-2.19

[halo-dev-bot]

@guqing: once the present PR merges, I will cherry-pick it on top of release-2.19 in a new PR and assign it to you.

In response to this:

/cherry-pick release-2.19

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[sonarqubecloud]

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]

Codecov Report

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

Project coverage is 58.06%. Comparing base (6cd8dc8) to head (36bb93e).
Report is 55 commits behind head on main.

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

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6597      +/-   ##
============================================
- Coverage     58.18%   58.06%   -0.12%     
- Complexity     3774     3916     +142     
============================================
  Files           651      670      +19     
  Lines         22125    22966     +841     
  Branches       1538     1575      +37     
============================================
+ Hits          12873    13335     +462     
- Misses         8641     9015     +374     
- Partials        611      616       +5     

☔ View full report in Codecov by Sentry.
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[JohnNiang]

/approve

[ruibaby]

/lgtm

[f2c-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JohnNiang, ruibaby

The full list of commands accepted by this bot can be found here.

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Needs approval from an approver in each of these files:

• OWNERS [JohnNiang,ruibaby]

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[halo-dev-bot]

@guqing: new pull request created: #6609

In response to this:

/cherry-pick release-2.19

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.