When you have required file element in webform,
and you try submit form without upload any file, you can see that clientside validation doesn't work,
and the error return from the server.
The problem caused from change that done on webform module
https://www.drupal.org/node/1609324#webform-required →
The webform component field named 'mandatory' changed to 'required'
I created patch to change it on clientside_validation_webform module, attached.
Fixed
1.0
Webform Validation
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Issue #2983286 by golds, DamienMcKenna: Validation doesn't work on...
Automatically closed - issue fixed for 2 weeks with no activity.