Fix extention long length of file name

[zhongwuzw]

New Pull Request Checklist

• I have read and understood the CONTRIBUTING guide

• I have read the Documentation

• I have searched for a similar pull request in the project and found none

• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

• I have added the required tests to prove the fix/feature I am adding

• I have updated the documentation (if necessary)

• I have run the tests and they pass

• I have run the lint and it passes (pod lib lint)

This merge request fixes / reffers to the following issues: ...

Pull Request Description

Fixes based on discussion in #2444 .

[codecov-io]

Codecov Report

Merging #2516 into master will increase coverage by 0.05%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #2516      +/-   ##
==========================================
+ Coverage   74.33%   74.38%   +0.05%     
==========================================
  Files          36       36              
  Lines        3951     3955       +4     
==========================================
+ Hits         2937     2942       +5     
+ Misses       1014     1013       -1

Impacted Files	Coverage Δ
SDWebImage/SDImageCache.m	64.34% <75%> (+0.5%)	⬆️
SDWebImage/SDWebImageManager.m	73.21% <0%> (-1.34%)	⬇️
SDWebImage/SDWebImageDownloaderOperation.m	88.42% <0%> (+0.96%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b5afc32...41214cc. Read the comment docs.

[bpoplauschi]

Looks good to me. Thanks @zhongwuzw for doing this PR.

[bpoplauschi]

Added 6c6d848 with 2 unit tests checking the two cases discussed in #2444