checkfile_http.js

var
    express = require('express'),
    gm = require('gm'),
    http = require('http'),
    request = require('request'),
    urlmod = require('url'),
    app = express.createServer();

app.use(express.logger('tiny'));
app.use(express.errorHandler({
    dumpExceptions: true,
    showStack: true
}));

function checkURL(options, callback) {
    // TODO: some kind of timeout would be nice
    var
        url = options.url,
        supportedTypes = options.supportedTypes,
        maxSize = options.maxSize;

    request.head(url, function(err, response, data) {
        if (err) {
            // Probably anything from a timeout to network error.
            console.error(err);
            callback({name: "ServerError"});

        } else if (response.statusCode != 200) {
            // Assume not found for now.
            // What about other success statuses?
            callback({name: "NotFound"});

        } else {
            var
                contentType = response.headers['content-type'],
                contentLength = response.headers['content-length'];

            if (supportedTypes.indexOf(contentType) == -1) {
                callback({name: "UnknownType"});

            } else if (!contentLength || contentLength < maxSize) {
                // looks OK so far
                callback(null, {
                    contentType: contentType,
                    contentLength: contentLength
                });

            } else {
                callback({name: "TooBig"});
            }
        }
    });
};

function identifyImageURL(options, callback) {
    // TODO:
    // * some kind of timeout would be nice
    // * and maybe a whitelist of supported image formats
    // * perhaps we should also make sure that only websafe features like
    //   colour profiles or modes are used
    var
        url = options.url,
        maxSize = options.maxSize;
        //maxSize = 0, // HACK
        total = 0;

    http.get(urlmod.parse(url), function(res) {
        res.on('data', function(chunk) {
            total += chunk.length;
            console.log(chunk.length, total);
            if (total > maxSize) {
                console.log('hanging up');
                res.destroy();
            }
        });
        var image = gm(res);
        image.identify(function(err, data) {
            if (err) {
                if (total > maxSize) {
                    callback({name: "TooBig"});
                } else {
                    callback({name: "ServerError"});
                }
            } else {
                if (total > maxSize) {
                    callback({name: "TooBig"});
                } else {
                    data.contentLength = total;
                    callback(null, data);
                }
            }
        });
    });
};

app.get('/check/image', function(req, res) {
    var
        url = req.query.url,
        maxSize = 5*1024*1024 // 5Mb

    checkURL({
        url: url,
        supportedTypes: ['image/png', 'image/jpeg', 'image/gif'],
        maxSize: maxSize
    },
    function(err, info) {
        //console.log("checkURL", err, data);
        if (err) {
            res.send({error: err.name});
        } else {
            identifyImageURL({
                url: url,
                maxSize: maxSize
            },
            function(err, features) {
                //console.log("identifyImageURL", err, features);
                if (err) {
                    res.send({error: err.name});
                } else {
                    data = {
                        contentType: info.contentType,
                        contentLength: features.contentLength,
                        format: features.format,
                        width: features.size.width,
                        height: features.size.height
                    };
                    res.send(data);
                }
            });
        }
    });
});

app.listen(3000);

A version of https://gist.github.com/2730750 using node's http module. This one works.