From my point of view on any member you want, as long as it is in your UI and you are using a microflow in order to show the validation message. If your microflow contains the activity "Show validation Message", you will be able to select the variable and the member.
As far as I know, the HasContents boolean gets set automatically to indicate whether there is an attachment uploaded or not... so you can use this field to determine whether to issue a warning or not