Find out if an attachment is embedded or attached

I'm not sure if this is a solution that is valid in all cases, but it works in my environment. That means "test it properly".

Const PR_ATTACH_CONTENT_ID = "http://schemas.microsoft.com/mapi/proptag/0x3712001F"

Function IsEmbedded(Att As Attachment) As Boolean
    Dim PropAccessor As PropertyAccessor
    Set PropAccessor = Att.PropertyAccessor
    IsEmbedded = (PropAccessor.GetProperty(PR_ATTACH_CONTENT_ID) <> "")
End Function

Call it with

If IsEmbedded(myAttachments(lngAttachmentCount)) Then
    ...
End If

The cryptic url-looking constant is not a url, but a property identifier. You can find a list of them here: https://interoperability.blob.core.windows.net/files/MS-OXPROPS/%5bMS-OXPROPS%5d.pdf

That property is set to the url of the attachment if embedded. If not embedded, then it is empty.

In the Outlook object model it's very important to marshal your objects correctly. Leaving a PropertyAccessor hanging about is not good, so I would suggest a minor modification to the accepted answer as follows:

Const PR_ATTACH_CONTENT_ID = "http://schemas.microsoft.com/mapi/proptag/0x3712001F"

Function IsEmbedded(Att As Attachment) As Boolean
    Dim PropAccessor As PropertyAccessor = Nothing
    Try
        PropAccessor = Att.PropertyAccessor
        Return (PropAccessor.GetProperty(PR_ATTACH_CONTENT_ID) <> "")
    Catch
        Return False
    Finally
        If PropAccessor IsNot Nothing
            Marshal.ReleaseCOMObject(PropAccessor)
        End If
    End Catch
End Function