I have 2 Passport entries in 1Password (me and my wife), each with an attachment (a photo). They were in my local vault. Using 1Password for Mac, I selected both of them, right-clicked, and chose to Move to the Shared vault of our team. Both entries moved over, but the attachment on one of the entries vanished. The attachment on the other was imported just fine, but the first attachment simply never got imported.
One potential complication: both attachments had the same name ("Passport Photo.jpg").
Thankfully, 1Password kept my old entries as items in the Trash, so I was able to rescue the photo from there.
1Password Version: 6.2.BETA-9
Extension Version: Not Provided
OS Version: OS X 10.11.4
Sync Type: Not Provided