I see 'interesting' behaviour when I search while attaching a record that without making a search I'm only able to attach one record but after making a search I'm offered multiple selection but only one of the selected records is attached.
From my Attachment View Page...
Would you allow these steps to attach the multiple records?
This is a know issue and we are planning to address this in the future releases.
The defect Id for this is 74900.We'll keep you posted on the fix release.
Thanks for reporting!