[BUG] upload plugin error

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

### Expected Behavior

_No response_

### Steps To Reproduce

1. plugin manager
2. add plugin
3. upload jar

### Environment

```markdown
ShenYu version(s): 2.6.1
```

### Debug logs

### Anything else?

I don't know what's wrong . so where is custom plugin detail document.
I read https://shenyu.apache.org/zh/blog/Loader-SourceCode-Analysis-ExtLoader
and https://shenyu.apache.org/zh/docs/developer/custom-plugin#%E5%8D%95%E4%B8%80%E8%81%8C%E8%B4%A3%E6%8F%92%E4%BB%B6

but i can't custom plugin success.

Reproduce the problem from Plugin Manager by adding a plugin and uploading its JAR, using the ShenYu version 2.6.1 environment and the attached debug logs. Read the linked custom-plugin documentation and compare its requirements with the upload flow. Done means the cause of the upload error is identified and a custom plugin can be uploaded successfully.

Written by the indexing model from the issue text.