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New features:
- Add support for the "accept" attribute on file inputs.
If the widget's field - if there is one - has the "accept" attribute set (the
`NamedImage` field has `image/*` set by default) then this is rendered as an
`accept` attribute on the file input.
This would restrict the allowed file types before uploading while still being
checked on the server side.
Fixes: https://github.com/plone/plone.formwidget.namedfile/issues/66
Depends on: https://github.com/plone/plone.namedfile/pull/158
[thet] (67)