[Request] Add file type filter to Asset field

[andrevvm]

It would be nice to be able to limit Asset fields by file type, either by MIME, extension, or categorized type (i.e. Kirby CMS).

[pauloamgomes]

Think that makes all sense, currently Cockpit permits to limit the upload (global) based on a whitelist rule in the configuration. However, that is not practical for some cases, e.g.:

• A document field would be probably limited to PDF files
• An image field would be limited to only common web formats

Such configuration should reside in the field definition, e.g.:

{
  "pattern": "*.png|*.jpg|*.pdf"
}