Attachment is uploading uisng GlideSysAttachment but error while downloading

[riyachuphal]

I have tried uploading my img using GlideSysAttachment() into my record and it uploads but when i try to view it - it shows file format not  supported. Please help me understand the fault in uploading process.
Here's my code for writing data into content variable:

upFile.addEventListener('change', attachFiles, false);
function attachFiles(evt){
	files = evt.target.files;
	for (var i = 0; i < files.length; i++) {
        var file = files[i];
        filename = file.name;
        contentType = file.type;
		var fileR = new FileReader();
        fileR.onload = (function(file) {
            return function(e) {
                content = e.target.result;
            };
        })(file);
		fileR.readAsDataURL(file);
	}
	document.getElementById("submit").disabled = false;
}

 Then a glideajax call to call my script include:

function onSubmit() 
{
	GlideDialogWindow.get().destroy();
	alert(content);
	var retestga = new GlideAjax("Myscripts");
		retestga.addParam("sysparm_name", "uploadRetestAttachments1");
		retestga.addParam("filename", filename);//filename);
		retestga.addParam("contenttype", contentType);//contentType);
		retestga.addParam("img_content", content);//window.btoa(content));
		retestga.addParam("rec_sysid", '0871******************');
		return true;
}

and here's the script include :

uploadRetestAttachments1 : function(){ 
		gs.addErrorMessage('hehhhhhhh');
		var fileName = this.getParameter('filename'); 
		var contentType = this.getParameter('contenttype'); 
		var content = this.getParameter('img_content');
		var itemSysId = this.getParameter('rec_sysid');
		gs.addInfoMessage('Processing file:  hello- '+fileName +" - "+contentType+content);
		if(typeof content =='string'){
			var data =content.split(',')[0];
			var data1 = content.split(',')[1];
			gs.addInfoMessage(data);
			gs.addInfoMessage(data1);
		}
		var attachment = new GlideSysAttachment();
		var rec = new GlideRecord("retest_table");
		rec.get(itemSysId);  
		var resp = attachment.write(rec, fileName, contentType, data1);
		gs.addInfoMessage('resp-----'+resp);
	return true;
},

I am able to see my encoded string correctly and if I try to decode it using some external online decoders- it works .

Here's the encoded string for reference:
iVBORw0KGgoAAAANSUhEUgAAAAEAAAADCAYAAABS3WWCAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAAFiUAABYlAUlSJPAAAAASSURBVBhXY/gPBEwMQAAkGBgAMfsEARoEDOwAAAAASUVORK5CYII=

Actual pic is just one small white dot and my error on download: