Hi, 

The file dropper widget uses mime types for restricting users to upload specific file types.

For example:  if you give image/* – this will allow all the image types like .jepg, .png, etc., So you have to remove image/* and replace it with image/jpeg and so on. 

Ensure you remove all the /* types mentioned by default and replace them with required Mime types. 

You can refer to all the mime types Here.

Hope this helps!

It looks like the file dropper widget has an option to add “verification before upload”

If you make sure your verification entity has an attribute for the extension, fill it out in “extension attr.” and then use it in your “before accept mf” to make sure the filetype is one of the ones that you allow.