chore: return 550 when file path is a dir in handleSIZE

[allape]

Adapt VLC FTP browser, see https://github.com/videolan/vlc/blob/f2379a0e276597efdfe7a6aa970fb0f9ae21fbc3/modules/access/ftp.c#L725

[drakkan]

Thanks for this contribution. It is ok for me. If you can, please add a simple test case, for example within the existing test for the SIZE command.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.68%. Comparing base (fef7c7b) to head (97608b4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #501      +/-   ##
==========================================
+ Coverage   86.66%   86.68%   +0.02%     
==========================================
  Files          11       11              
  Lines        2324     2328       +4     
==========================================
+ Hits         2014     2018       +4     
  Misses        236      236              
  Partials       74       74              

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[allape]

Sure! And I add the test case for file in folder, which makes func ftpUpload broken in file transfer_test.go, and I add the secondary condition to make it work