Allow Arcade "Attachments" Function to Access Keywords

[jcarlson]

When I look at an attachment at service-url/FeatureServer/0/1/attachments, I see this:

But when I access the features in an Arcade expression, I see this:

I don't see any particular reason not to include the keywords, those they are not listed in the Data Types page of the documentation. Please add this capability! We have some layers with all kinds of attachments coming in, and I would like to be able to filter them for certain use cases.