Hi Andrei and welcome to the forum!
When you say Attachments collection are you referring to some repeating section in your data source that stores attachments? If so, then this will be populated by InfoPath when users fill out hte form and attach files to it. If not, then I'm confused and there is some other kind of attachment. Please help me understand.
It is possible that you have found a bug with WSE or InfoPath or both. We had issues with WSE and IP back a year or two ago because it only supports a certain kind of SOAP (I forget which one). Anyway, good to see you in the forum. We have resources and hope to help you solve your problem.
Patrick Halstead [InfoPath MVP]
InfoPathDev
Patrick Halstead
Project Manager at Qdabra