Invalid extension alert and clear input

[tlshifat]

Signed-off-by: Md Shifaul Islam shifat.it@gmail.com

Description

Invalid extension alert and clear input to upload valid files again.

Fixes #15541

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[tlshifat]

Please proceed . Existing flow is ok . Just an alert given for invalid extension.
Hope this will help.

[williamdes]

Please do a php server side check

But this check is restricting users as @ibennetch said

[codecov]

Codecov Report

Merging #15595 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master   #15595   +/-   ##
=========================================
  Coverage     52.98%   52.98%           
  Complexity    14051    14051           
=========================================
  Files           476      476           
  Lines         61362    61362           
=========================================
  Hits          32515    32515           
  Misses        28847    28847

[williamdes]

Thank you for the server side fix !

[williamdes]

Sorry but I will not merge this PR
IMO users do what they want and if they upload bad files no problem if it does not introduce a security issue.

I hope that I did not misunderstand something.