Upload file

 Upload attachment

ClientContext context = new ClientContext("http://sp2013");
List list = context.Web.Lists.GetByTitle("Test");
context.Load(list);
context.ExecuteQuery();

var item = list.AddItem(new ListItemCreationInformation());
item["Title"] = DateTime.Now.ToString("yyyy-MM-dd HH:mm");
item.Update();
context.ExecuteQuery();

string fileName = "attachment.png";
string base64Str = "data:image/png;base64,iVBORw0KGgoABJRU5ErkJggg==";
var attInfo = new AttachmentCreationInformation();
attInfo.FileName = fileName;          
byte[] arr;
string convert = base64Str.Split(new string[] { ";base64," }, StringSplitOptions.None)[1];
arr = Convert.FromBase64String(convert);
attInfo.ContentStream = new MemoryStream(arr);

var att = item.AttachmentFiles.Add(attInfo);
context.Load(att);
context.ExecuteQuery();
Console.WriteLine(att.ServerRelativeUrl);

//byte[] filecontent= (byte[])reader[0];

Upload file to Library

byte[] filecontent = testItem.filecontent;
string fileName = testItem.filename;
Folder folder = list.RootFolder;
FileCreationInformation fileCreate = new FileCreationInformation();
fileCreate.Content = filecontent;
string strUrl=web.Url+ "/Shared%20Documents/" + fileName;
fileCreate.Url = strUrl;
folder.Files.Add(fileCreate);
web.Update();
context.ExecuteQuery();