建议使用附件上传接口方法：

AttachmentServerManager manager = AttachmentManagerFactory.getServerManager(ctx)

manager.addNewAttachmentContainCtx()

demo:

File fin=new File("D:/test/testtt.pdf");
FileInputStream in=new FileInputStream(fin);
//得到输入流
//InputStream inputStream = conn.getInputStream();
AttachmentServerManager manager = AttachmentManagerFactory.getServerManager(ctx);
ByteArrayOutputStream baot = new ByteArrayOutputStream();
byte[] bytes = new byte[1024]; //定义一个数组 用来读取
while((in.read(bytes)) != -1){
baot.write(bytes); //将读取的字节流写入字节输出流
}
byte[] outbyte = baot.toByteArray();//将字节输出流转为自己数组。
in.close();
baot.close();
SimpleAttachmentInfo simple = new SimpleAttachmentInfo();
simple.setContent(outbyte);
simple.setMainName("testAttachment11"); //文件名
simple.setExtName("pdf"); //后缀
manager.addNewAttachmentContainCtx(ctx, "需要上传到单据的fid", simple);

注：以上demo代码仅供参考