If you change the attachment datagrid to a source microflow (instead of database), you will be able on that microflow source to retrieve the attachment with the email put in the RequestorEmailAddress.
When a user is clicking on a microflow button (caption : refresh for example), you will refresh the object accessRequest, this refresh will trigger the microflow source of the datagrid.
By the way, if you only want to retrieve one attachment, a dataview instead of a datagrid will be better.