Detecting if an image is really an image

async.coffee

request = new XMLHttpRequest()
request.onreadystatechange = ->
    if request.readyState is 4
        mimetype = request.getResponseHeader('content-type')

        if mimetype.split('/').shift() is 'image'
            alert 'Image!'
        else
            alert 'Not Image!'
request.open 'HEAD', 'http://upload.wikimedia.org/wikipedia/commons/5/5b/Ultraviolet_image_of_the_Cygnus_Loop_Nebula_crop.jpg', true
request.send()

sync.coffee

request = new XMLHttpRequest()
request.open 'HEAD', 'http://upload.wikimedia.org/wikipedia/commons/5/5b/Ultraviolet_image_of_the_Cygnus_Loop_Nebula_crop.jpg', false
request.send()

mimetype = request.getResponseHeader('content-type')
if mimetype.split('/').shift() is 'image'
    alert 'Image!'
else
    alert 'Not Image!'