MAM policies - attach from photo gallery

Hi all:
I am testing both iOS and Android policies and have the following set in both (or as similar):

Receive data from other apps = Policy Managed Apps

So I know that when composing or replying to an email with the Outlook Mobile app, I am restricted as to where I can attach a file from.  However, I can attach from the device's local photo gallery without issue.  Why when the policy restricts receiving data to only policy managed apps?

Is attaching from the photo gallery different than attaching a file?

Paul