Hi Tosha,
I have sent a sample form to your email and also attaching a sample here regarding this issue.
concat("GetAttachmentAttribute /xpath=/my:myFields/my:Attachments/my:Attachment[", count(preceding-sibling::Attachment) + 1, "]/my:File /attr=Name")
I believe that the sample formula above is correct, but I'm not quite sure why it is not working for you. If it is not the command syntax, then I am guessing that the form's arrangement is causing the issue. Please check the form I am attaching for your reference. Hope this helps.