How to prevent a form submission without an attachment?

[impin]

need to validate before the form submit, whether the user selected the file for upload...

without file attachment the form shouldn't be submitted. it will show a message to the user "please a select file to upload'...

how to do it with javascript??

its my form...

<form method="POST" name="email" action="sendmail.php" enctype="multipart/form-data" onSubmit="return validateForm()";>

    <table border=0 cellPadding=5 cellSpacing=10 width=100%>
   
        <tr>
            <td><label for='name'>Name:</label></td>
            <td><input  type="text" name="name"  maxlength="50" size="30" ononkeyup="AllowAlphabet()"></td>
        </tr>
         <tr>
            <td><label for='familyname'>Family Name:</label></td>
            <td><input  type="text" name="fname" maxlength="50" size="30"></td>
        </tr>
         <tr>
            <td><label for='age'>Age:</label></td>
            <td><input  type="number" name="age" maxlength="50" size="30"></td>
        </tr>
        <tr>
            <td><label for='experience'>Years of Experience:</label></td>
            <td><input  type="number" name="exp" maxlength="50" size="30"></td>
        </tr>
        <tr>
            <td><label for='university'>University Attened:</label></td>
            <td><input  type="text" name="university" maxlength="50" size="30"></td>
        </tr>
         <tr>
            <td><label for='phone'>Phone Number:</label></td>
            <td><input  type="text" name="phone" maxlength="50" size="30"></td>
        </tr>
         <tr>
            <td><label for='email'>Email:</label></td>
            <td><input  type="email" name="email" maxlength="50" size="30"></td>
        </tr>
        <tr>
            <td><label for='resume'>Resume:</label></td>
            <td><input type="file" name="resume"></td>
        </tr>
        <tr>
            <td colSpan=2 align=center><input class=subbtn type="submit" value="Submit" name='submit' /></td>
        </tr>
        
    </table>
</form>

[Dormilich]

just check, if the appropriate input field is empty (has an empty value).