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

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.55%. Comparing base (9c2ba6e) to head (5e126b6).

Files with missing lines	Patch %	Lines
handle_files.go	40.00%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #501      +/-   ##
==========================================
- Coverage   86.66%   86.55%   -0.11%     
==========================================
  Files          11       11              
  Lines        2324     2328       +4     
==========================================
+ Hits         2014     2015       +1     
- Misses        236      237       +1     
- Partials       74       76       +2     

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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