Maybe this is bug of iReport, just maybe.
While open an report xml file using iReport, do edit, then save, most time it works well.
But if there is some blob fields using java.io.InputStream type, via doing edit, compile all ok, but doesnot works.
Error,en, what I remembered is "ClassCast error".
So, by checking xml, I founde iReport change java.io.InputStream to java.lang.Object;
In jasperreport, there is no the ability to doing the Class Type Conversation.