[Ex_Soft]

me.imgDownloadButton = Ext.create('Ext.button.Button', {
              text : nls('Download'),
              handler :function() {
                  window.location.href = me.getAddress() +"&chartW="  +100  +"&chartH="  +100;
              }
          });

и отдавайте картинку с

context.Response.ContentType = "application/octet-stream";

или

...
			handler: function(btn, e) {
				var
					ifrm,
					ifrmId="ifrDownload";

				if(!(ifrm=Ext.getDom(ifrmId)))
					Ext.DomHelper.append(Ext.getBody(), {
						tag: "iframe",
						id: ifrmId,
						frameBorder: 0,
						style: "display: none; height: 1px; width: 1px; ",
						src: me.getAddress() +"&chartW="  +100  +"&chartH="  +100;
					})
				else
					ifrm.src=me.getAddress() +"&chartW="  +100  +"&chartH="  +100;
			},
...