I found that functionality of disallow file formats doesn't work if file upload field allows multiple values. It requires some corrections in module fff and maybe even in Drupal Core.

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eugene.ilyin created an issue. See original summary.

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eugene.ilyin CreditAttribution: eugene.ilyin as a volunteer and at DrupalJedi commented
Title: Doesn't work with multiple values field » Functionality of disallow file formats doesn't work if file upload field allows multiple values
Issue summary: View changes

  • eugene.ilyin committed 7063eb8 on 8.x-1.x 
    Issue #2594851 by eugene.ilyin: Functionality of disallow file formats...
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eugene.ilyin CreditAttribution: eugene.ilyin as a volunteer and at DrupalJedi commented

I've corrected this problem but for each render of the element will be notice
Notice: Undefined offset: 0 in template_preprocess_file_upload_help() (line 186 of core/modules/file/file.field.inc).

It can be solved in related issue #2594389.

If somebody can propose an alternative solution, I would be happy!