fix(upload): preview can't show image when the image file use uppercase extension.

[sliwey]

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Upload preview can't show image when the image file use uppercase extension, like .JPG
This pr fix it, and remove unnecessary escape character in function extname.

[ant-design-bot]

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Built with commit f614668

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[codecov]

Codecov Report

Merging #10928 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #10928   +/-   ##
=======================================
  Coverage   91.59%   91.59%           
=======================================
  Files         194      194           
  Lines        4769     4769           
  Branches     1329     1329           
=======================================
  Hits         4368     4368           
  Misses        397      397           
  Partials        4        4

Impacted Files	Coverage Δ
components/upload/UploadList.tsx	88.77% <100%> (ø)	⬆️

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