Fix exception when uploading a file without an extension

[SapientGuardian]

Fix UploadButton to use System.IO.Path.GetExtension instead of Substring to support filenames without extensions

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• Component style update
• Bundle size optimization
• Performance optimization
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

#3553

💡 Background and solution

Uploading a file that does not have a file extension using the Upload[button] component results in the following exception being thrown:

blazor.webassembly.js:1 crit: Microsoft.AspNetCore.Components.WebAssembly.Rendering.WebAssemblyRenderer[100]
      Unhandled exception rendering component: ArgumentOutOfRange_Generic_MustBeNonNegative, startIndex, -1 Arg_ParamName_Name, startIndex
      ArgumentOutOfRange_ActualValue, -1
System.ArgumentOutOfRangeException: ArgumentOutOfRange_Generic_MustBeNonNegative, startIndex, -1 Arg_ParamName_Name, startIndex
ArgumentOutOfRange_ActualValue, -1
   at AntDesign.Internal.UploadButton.FileNameChanged(ChangeEventArgs e)
   at Microsoft.AspNetCore.Components.ComponentBase.CallStateHasChangedOnAsyncCompletion(Task task)
   at Microsoft.AspNetCore.Components.RenderTree.Renderer.GetErrorHandledTask(Task , ComponentState )

This is happening because the UploadButton component uses Substring based on the position of the last ., which is -1 when no . is present. This PR fixes this by using the built-in System.IO.Path.GetExtension method instead, which better expresses intent and is not affected by this bug.

There are no existing test cases covering this code path and I struggled to create one, but would be happy to add one with assistance.

📝 Changelog

Language	Changelog
🇺🇸 English	Fixed exception when uploading a file without an extension
🇨🇳 Chinese	修复了上传没有扩展名的文件时出现的异常

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• Changelog is provided or not needed

[github-actions]

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (316d0aa) 44.38% compared to head (9f37487) 1.53%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
components/upload/UploadButton.razor.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #3554       +/-   ##
==========================================
- Coverage   44.38%   1.53%   -42.85%     
==========================================
  Files         566     566               
  Lines       27174   27114       -60     
  Branches     5576    5554       -22     
==========================================
- Hits        12061     417    -11644     
- Misses      14128   26657    +12529     
+ Partials      985      40      -945     

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[ElderJames]

Thanks for contribution @SapientGuardian , this helps a lot! And it would be nice to add tests.

[SapientGuardian]

I've updated the PR with a test and changed the commit message to match the standard format.

[ElderJames]

Thank you @SapientGuardian