Fix FilesHandler not meet RFC 6713

[Wh1isper]

just like https://github.com/tornadoweb/tornado/blob/master/tornado/web.py#L2896

We do some modify here https://github.com/jupyter-server/jupyter_server/blob/main/jupyter_server/base/handlers.py#L745

but forget FilesHandler

[codecov-commenter]

Codecov Report

Merging #701 (e5558eb) into main (745f5ba) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #701      +/-   ##
==========================================
- Coverage   77.97%   77.95%   -0.03%     
==========================================
  Files         115      115              
  Lines       10817    10821       +4     
  Branches     1465     1467       +2     
==========================================
  Hits         8435     8435              
- Misses       1981     1985       +4     
  Partials      401      401              

Impacted Files	Coverage Δ
jupyter_server/files/handlers.py	33.33% <0.00%> (-2.52%)	⬇️

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[blink1073]

Thanks!