Hi,
As Stefan already mentioned it is default behavior of SOAP adapter, hence you should look for work around options.
One work around would be along with attachment source system should send file Name in httpheaders e.g., such as Dynamicconfiguration header in the SOAP message. In mapping read the header file name and then create a new attahcment by reading the incoming attachment content. Finally remove the incoming original attachment if required in the mapping. ***Please note that remove attachment method might not work for all PI versions.
**Update:
There is no need even for fileName in http header (e.g., as dynamic configuration) as mentioned above. Just read the contentID of the incoming attachment should give the actual fileName. Then create a fresh attachment using the read contentID as name with byte content of incoming attachment. Remove the original attachment if required as mentioned above.
Regards,
Praveen Gujjeti