Validation for filename extensions doesn't seem to work

[dannyuk1982]

Describe the bug
Validation for filename extensions for type assets don't seem to work

To Reproduce

1. Add a validate line to an assets field, i.e.

  container: main
  folder: listings/thumbnails
  restrict: true
  max_files: 1
  type: assets
  validate: "ext:jpg,gif"

2. Select a valid file, i.e. ending in .jpg
3. Submit the Workshop form

Expected behavior
I expect the file to be accepted and saved

Actual behavior
An error occurs The Thumbnail must be a file of type: jpg, gif and the image is not saved. Removing the validate line is needed for the image to upload - which is of no use.

Environment details (please complete the following information):

• Workshop Version 1.1.0
• Statamic Version 2.11.12