adding deny option to filepicker

[ricardo118]

This option is the reverse of accept.

This is useful to remove the item.json from the filepicker without having to set any accepted.

Example:

To create a filepicker that accepts images and videos and files youd need a very large accept:, instead you can just have a

deny:
 - .json

and allow everything else

[mahagr]

I guess this is fine, though I would love to see some global option to deny certain extensions regardless of what is allowed in the field.