I compared the output pdfs between version 0.7.3 and version 1.6.0. The 1.6.0 one has the following extra statement under the EmbeddedFilesNameTreeNode obj :
/Limits [(My first attachment) (My first attachment)]
It seems that this line is the cause of the problem. However I couldn't find any API to get rid of this line, so I copied the setNames method in org.apache.pdfbox.pdmodel.common.PDNameTreeNode and modified it to a new method :
and called this method instead of efTree.setName() in the example. It seems solve the problem and works fine in my project. But I am new to pdf so that I don't know whether it will cause any side effect.