Salesforce apex to send image file to 3rd party using REST API

//attObj is an Object of Attachment object for which values will be coming from Visualforce page.
//Attachment attObj = new attachment();

public pagereference customsave(){
String fileName = attObj.name;
Blob file_body = attObj.body;
String targetURL = system.label.ImageUploadEndPointURL; //API End point URL

String boundary = ‘—————————-741e90d31eff’;

// assemble the body payload
String header = ‘–‘+boundary+’\nContent-Disposition: form-data; name=”file”;
filename=”‘+fileName+'”;\nContent-Type: application/octet-stream’;
String footer = ‘–‘+boundary+’–‘;
String headerEncoded = EncodingUtil.base64Encode(Blob.valueOf(header+’\r\n\r\n’));
while(headerEncoded.endsWith(‘=’))
{
header+=’ ‘;
headerEncoded = EncodingUtil.base64Encode(Blob.valueOf(header+’\r\n\r\n’));
}
String bodyEncoded = EncodingUtil.base64Encode(file_body);
Blob bodyBlob = null;
String last4Bytes = bodyEncoded.substring(bodyEncoded.length()-4,bodyEncoded.length());

if(last4Bytes.endsWith(‘==’)) {
last4Bytes = last4Bytes.substring(0,2) + ‘0K’;
bodyEncoded = bodyEncoded.substring(0,bodyEncoded.length()-4) + last4Bytes;
// We have appended the \r\n to the Blob, so leave footer as it is.
String footerEncoded = EncodingUtil.base64Encode(Blob.valueOf(footer));
bodyBlob = EncodingUtil.base64Decode(headerEncoded+bodyEncoded+footerEncoded);
}else if(last4Bytes.endsWith(‘=’)) {
last4Bytes = last4Bytes.substring(0,3) + ‘N’;
bodyEncoded = bodyEncoded.substring(0,bodyEncoded.length()-4) + last4Bytes;
// We have appended the CR e.g. \r, still need to prepend the line feed to the footer
footer = ‘\n’ + footer;
String footerEncoded = EncodingUtil.base64Encode(Blob.valueOf(footer));
bodyBlob = EncodingUtil.base64Decode(headerEncoded+bodyEncoded+footerEncoded);
} else {
// Prepend the CR LF to the footer
footer = ‘\r\n’ + footer;
String footerEncoded = EncodingUtil.base64Encode(Blob.valueOf(footer));
bodyBlob = EncodingUtil.base64Decode(headerEncoded+bodyEncoded+footerEncoded);

}

HttpRequest req = new HttpRequest();
req.setHeader(‘Content-Type’,’multipart/form-data; boundary=’+boundary);
req.setMethod(‘POST’);
req.setEndpoint(targetURL);
req.setBodyAsBlob(bodyBlob);
req.setTimeout(120000);

Http http = new Http();

HTTPResponse res = http.send(req);
System.debug(‘res.getBody()’ + res.getBody());
}

//VisualForce Page