Usando las funciones de macromedia sería así:
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>
<script type="text/JavaScript">
<!--
function MM_findObj(n, d) { //v4.01
var p,i,x; if(!d) d=document; if((p=n.indexOf("?"))>0&&parent.frames.length) {
d=parent.frames[n.substring(p+1)].document; n=n.substring(0,p);}
if(!(x=d[n])&&d.all) x=d.all[n]; for (i=0;!x&&i<d.forms.length;i++) x=d.forms[i][n];
for(i=0;!x&&d.layers&&i<d.layers.length;i++) x=MM_findObj(n,d.layers[i].document);
if(!x && d.getElementById) x=d.getElementById(n); return x;
}
function MM_validateForm() { //v4.0
var i,p,q,nm,test,num,min,max,errors='',args=MM_validateForm.arguments;
for (i=0; i<(args.length-2); i+=3) { test=args[i+2]; val=MM_findObj(args[i]);
if (val) { nm=val.name; if ((val=val.value)!="") {
if (test.indexOf('isEmail')!=-1) { p=val.indexOf('@');
if (p<1 || p==(val.length-1)) errors+='- '+nm+' must contain an e-mail address.\n';
} else if (test!='R') { num = parseFloat(val);
if (isNaN(val)) errors+='- '+nm+' must contain a number.\n';
if (test.indexOf('inRange') != -1) { p=test.indexOf(':');
min=test.substring(8,p); max=test.substring(p+1);
if (num<min || max<num) errors+='- '+nm+' must contain a number between '+min+' and '+max+'.\n';
} } } else if (test.charAt(0) == 'R') errors += '- '+nm+' is required.\n'; }
} if (errors) alert('The following error(s) occurred:\n'+errors);
document.MM_returnValue = (errors == '');
}
//-->
</script>
</head>
<body>
<form name="alojar" method="post" action="http://url/includes/upload_basic.php" style="margin:0px;" enctype="multipart/form-data" onsubmit="MM_validateForm('theme','','R','preview','','R','nombre','','R'); return document.MM_returnValue;">
<table width="770" border="0" cellspacing="0" cellpadding="0">
<tr>
<td width="400" height="20" align="left">Imagen preview: </td>
<td width="30" align="left"> </td>
<td align="left">Nombre:</td>
<td align="left"> </td>
</tr>
<tr>
<td height="20" align="left"><input name="preview" type="file" size="40" /></td>
<td align="left"> </td>
<td align="left"><input name="nombre" type="text" size="25" /></td>
<td align="left"> </td>
</tr>
<tr>
<td height="30" align="left" valign="bottom">Theme:</td>
<td align="left"> </td>
<td align="left" valign="bottom"> </td>
<td align="left" valign="bottom"> </td>
</tr>
<tr>
<td align="left"><input name="theme" type="file" size="40" /></td>
<td colspan="2" rowspan="2" align="left"><span style="text-align:center; margin:10px;">
<input name="enviar" type="image" style="cursor:pointer" src="http://url/media/images/formulario/upload.gif" border="0"/>
</span></td>
<td align="left" valign="bottom"> </td>
</tr>
<tr>
<td align="left"> </td>
<td align="left" valign="bottom"> </td>
</tr>
</table>
</form></body>
</html>