I haven't tried it, but you can see the metadata for the Unified API by going to https://graph.microsoft.com/beta/$metadata. From looking at that, it seems the structure of the Attachment
entity is the same as the Attachment
entity in the Outlook APIs.
The relevant bits of the metadata:
<EntityType Name="Attachment">
<Key>
<PropertyRef Name="Id"/>
</Key>
<Property Name="Id" Unicode="false" Type="Edm.String" Nullable="false"/>
<Property Name="Name" Unicode="false" Type="Edm.String"/>
<Property Name="ContentType" Unicode="false" Type="Edm.String"/>
<Property Name="Size" Type="Edm.Int32" Nullable="false"/>
<Property Name="IsInline" Type="Edm.Boolean" Nullable="false"/>
<Property Name="DateTimeLastModified" Type="Edm.DateTimeOffset"/>
</EntityType>
<EntityType Name="FileAttachment" BaseType="Microsoft.Graph.Attachment">
<Property Name="ContentId" Unicode="false" Type="Edm.String"/>
<Property Name="ContentLocation" Unicode="false" Type="Edm.String"/>
<Property Name="IsContactPhoto" Type="Edm.Boolean" Nullable="false"/>
<Property Name="ContentBytes" Type="Edm.Binary"/>
</EntityType>
<EntityType Name="ItemAttachment" BaseType="Microsoft.Graph.Attachment">
<NavigationProperty Name="Item" Type="Microsoft.Graph.OutlookItem" Nullable="false" ContainsTarget="true"/>
</EntityType>