Kanasansoft Web Lab.

for(var i=0;i<256;i++){
    var str=String.fromCharCode(i);
    console.log(str,str.charCodeAt(0),str.charCodeAt(0)==i,str.charCodeAt(0)==0xFFFD);
}

GM_xmlhttpRequest(
    {
        "method":"GET",
        "url":url,
        "onload":callbackForSuccess,
        "onerror":callbackForError,
        "headers":{
            "If-Unmodified-Since":lastModified,
            "Range":"bytes="+from+"-"+To
        }
    }
);

// ==UserScript==
// @name        getFileByDataScheme
// @namespace   http://www.kanasansoft.com/
// @include     *
// ==/UserScript==

(
    function(){

        var base64=function(original){
            var table=(
                function(s){
                    var len=s.length;
                    var hash={};
                    for(var i=0;i<len;i++){
                        var bit=i.toString(2);
                        var bit="000000".slice(0,-bit.length%6)+bit;
                        hash[bit]=s.charAt(i);
                    }
                    return hash;
                }
            )("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/");
            var len=original.length;
            var bits8=[];
            if(original instanceof Array){
                for(var i=0;i<len;i++){
                    var bit=original[i].charCodeAt(0).toString(2);
                    bit="00000000".slice(0,-bit.length%8)+bit;
                    Array.prototype.push.apply(bits8,bit.match(/.{8}/g));
                }
            }else{
                for(var i=0;i<len;i++){
                    var bit=original.charCodeAt(i).toString(2);
                    bit="00000000".slice(0,-bit.length%8)+bit;
                    Array.prototype.push.apply(bits8,bit.match(/.{8}/g));
                }
            }
            var len=bits8.length;
            var bits6=[];
            for(var i=0;i<len;i+=3){
                var bit=bits8.slice(i,i+3).join("");
                bit+="000000".slice(0,-bit.length%6);
                Array.prototype.push.apply(bits6,bit.match(/.{6}/g));
            }
            var len=bits6.length;
            var base64=[];
            for(var i=0;i<len;i++){
                base64.push(table[bits6[i]]);
            }
            Array.prototype.push.apply(base64,["=","=","=","="].slice(0,-base64.length%4));
            return base64.join("");
        }

        var parseHTTPHeader=function(responseHeader){
            var headers=responseHeader.split("\n");
            var len=headers.length;
            var parsing=[];
            for(var i=0;i<len;i++){
                if(/^$/.test(headers[i])){
                }else if(/^[\x09\x20]/.test(headers[i])){
                    if(parsing.length==0){
                        throw "SyntaxError:HTTPHeader (first line) "+headers[i];
                    }
                    parsing[parsing.length-1]+="\n"+headers[i];
                }else{
                    parsing.push(headers[i]);
                }
            }
            var len=parsing.length;
            var parsed={};
            for(var i=0;i<len;i++){
                var pair=parsing[i].split(": ",2);
                if(pair.length!=2){
                    throw "SyntaxError:HTTPHeader (format) "+parsing[i];
                }
                if(pair[0] in parsed){
                    throw "SyntaxError:HTTPHeader (repetition) "+pair[0];
                }
                parsed[pair[0]]=pair[1];
            }
            return parsed;
        }

        var getDataMethod=function(res){
            if(data.errorFlag){
                return;
            }
            if(res.status!=206){
                data.errorFlag=true;
                throw "HTTPStatusError (The status is not 206.)"+
                    ":status="+res.status+
                    ":statusText="+res.statusText+
                    ":responseHeaders="+res.responseHeaders;
            }
            var headers=parseHTTPHeader(res.responseHeaders);
            var contentRange=headers["Content-Range"];
            if(!contentRange){
                data.errorFlag=true;
                throw "Error:Sorry. The server do not support HTTP header of \"Content-Range\".";
            }
            var range=contentRange.match(/^bytes (\d+)-\d+\/\d+$/)[1];
            data.gottenData[parseInt(range,10)]=res.responseText;
            data.restData=data.restData.filter(
                (function(range){
                     return function(val){return range!=val;}
                 })(range)
            );
            if(data.restData.length==0){
                var contentType=("Content-Type" in headers)?
                    headers["Content-Type"]:
                    "application/octet−stream";
                var dataScheme="data:"+contentType+
                    ";base64,"+base64(data.gottenData);
                data.callback(dataScheme);
            }
        }

        var getDataByRangeHandler=function(range,callback){
            return function(){
                if(data.errorFlag){
                    return;
                }
                GM_xmlhttpRequest(
                    {
                        "method":"GET",
                        "url":data.url,
                        "onload":callback,
                        "onerror":callback,
                        "headers":{
                            "If-Unmodified-Since":data.lastModified,
                            "Range":"bytes="+range+"-"+range
                        }
                    }
                );
            }
        }

        var startGetData=function(){
            var len=data.restData.length;
            for(var i=0;i<len;i++){
                var getDataByRange=
                    getDataByRangeHandler(data.restData[i],getDataMethod);
                setTimeout(getDataByRange,10*i);
            }
        }

        var loadHeaderMethod=function(res){
            var headers=parseHTTPHeader(res.responseHeaders);
            if(res.status!=200){
                throw "RequestError"+
                    ":status="+res.status+
                    ":statusText="+res.statusText+
                    ":responseHeaders="+res.responseHeaders;
            }
            var headers=parseHTTPHeader(res.responseHeaders);
            var lastModified=headers["Last-Modified"];
            if(!lastModified){
                throw "Error:Sorry. The server do not support HTTP header of \"Last-Modified\".";
            }
            var contentLength=headers["Content-Length"];
            if(!contentLength){
                throw "Error:Sorry. The server do not support HTTP header of \"Content-Length\".";
            }
            contentLength=parseInt(contentLength,10);
            var restData=[];
            for(var i=0;i<contentLength;i++){
                restData.push(i);
            }
            data.lastModified=lastModified;
            data.size=contentLength;
            data.restData=restData;
            startGetData();
        }

        var getHeader=function(url){
            data.url=url;
            GM_xmlhttpRequest(
                {
                    "method":"HEAD",
                    "url":data.url,
                    "onload":loadHeaderMethod,
                    "onerror":loadHeaderMethod
                }
            );
        }

        var data={"errorFlag":false,"gottenData":[]};

        var getFileByDataScheme=function(url,callback){
            data.callback=callback;
            getHeader(url);
        }

        if(window.self==window.top){
            var url=prompt("image url","");
            if(url==null||url==""){
                return;
            }
            var handler=function(url,callback){
                return function(){
                    getFileByDataScheme(url,callback);
                }
            }
            var callback=function(dataScheme){
                var elem=document.createElement("img");
                elem.src=dataScheme;
                document.body.appendChild(elem);
            }
            window.addEventListener(
                "load",
                handler(url,callback),
                false
            );
        }

    }
)();