Salvo en Firefox 3, que permite hacer cosas como esta:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
</head>
<body><form action="" method="post" enctype="multipart/form-data" name="form1" id="form1">
<input type="file" name="file" onchange="document.getElementById('prev').style.display='inline';document.getElementById('prev').src='data:'+this.files.item(0).getAsDataURL();document.getElementById('fs').innerHTML=' '+(parseInt(this.files.item(0).fileSize)/1024).toFixed(2)+' kb';document.getElementById('log').innerHTML=this.files.item(0).getAsDataURL()" />
<img id="prev" src="" width="85" style="display:none;" /><span id="fs"></span>
</form>
<div id="log"></div>
</body>
</html>