I am not good at java and zimbra.
I have just found a solution about base64-encoded attachments but i couldn't find how i can use this codes in the system.
I think, i have to create a java program as zimlet but i don't know anything about it.
Could anyone please create completely zimlet for me to decode base64 attachments.

var jspUrl = this.getResource("attachmentToBase64.jsp")+"?messa geUrl="+AjxStringUtil.urlComponentEncode(attachmen t.url);

var response = AjxRpc.invoke(null, jspUrl, null, null, true, null);