Hi Maciej,
I had checked jdk-1.5.0_15 and jdk-1.5.0_22. There is no javax.activation package in both of them. The reason of this error is caused by activation-1.1 didn't handle byte[]. The activation-1.1.1 will check the data type, if there is a byte[], activation-1.1.1 will not find additional ContentDataHandler.
I can't find either activation-1.1.1 or ContentDataHandler configuration for image/gif. If you have time, could you add javax.activation.level=FINEST in the logging.properties and run AttachmentTest again, and show me the content of target/surefire-reports/org.jbpm.test.activity.mail.AttachmentTest-output.txt. It will help me to find what I missed. Furthermore could you show me your the result of 'java -version' and 'mvn depedency:tree'? Thank you very much.