Create icon.php in your app/view/helpers directory and fill in the following code:

<?php
/**
 * Class IconHelper
 * Retrieves an appropriate icon by given mimetype or file extension.
 */
class IconHelper extends AppHelper {
    var $name = "Icon";
    
    var $helpers = array( "Html" );

    /**
     * Path to fileicons in app/wwwroot/img/
     * @var String
     */
    var $pathFileicons = "fileicons";

    function setFileiconsPath( $pathFileicons ) {
        $this->pathFileicons = $pathFileicons;
    }
    
    function get( $type_or_ext, $extension = "" ) {
        $mimetype = "";
        
        // parse type_or_ext
        // if $extension is set, first param is interpreted as mimetype
        if( !empty( $extension ) ) $mimetype = $type_or_ext;
        // else if first param contains slash it is interpreted as mimetype
        elseif( strpos( $type_or_ext, "/" ) !== false )
            $mimetype = $type_or_ext;
        else     
            $extension = $type_or_ext;
        
        $mediatype = $subtype = "";
        
        if( !empty( $mimetype ))
            list( $mediatype, $subtype ) = explode("/", $mimetype);
        
        if( substr( $subtype, 0, 2) == "x-" ) $subtype = substr( $subtype, 2 );
        
        $try = array( $subtype, $extension, $mediatype, "file" );
        
        foreach( $try as $name )
            if( !empty( $name))
                if( ( $extension = $this->iconExists($name) ) !== false ) 
                    return $this->Html->image( $this->pathFileicons . DS . $name . ".$extension", 
                                               array( "alt" => $name ));
    }
    
    /**
     * Checks whether an icon of the given name exists in 
     * folder pathFileicons. Tries out several extensions.
     * @param $name
     * @return mixed: 
     * - false if no icon was found
     * - String $extension of the found icon
     * 
     */
    private function iconExists( $name ) {
        $path = IMAGES . DS . $this->pathFileicons . DS . $name;
        $extensions = array( "png", "gif", "ico", "jpg", "jpeg" );
        
        foreach( $extensions as $extension )     
            if( @file_exists( $path . ".$extension" ))
                return $extension;
                
        return false;
    }

}?>
Of course, add 'Image' to your $helpers property of your Controller class.

<?php
var $helpers = array( 'Image' );
?>
You need to create a folder with file icons in your images-folder (default: "app/webroot/img/fileicons/"). The icons should be named after the file extension, its media type or it subtype without leading "x-" and be of the type PNG, GIF, ICO or JPG. For instance, the following file icon names are possible:

pdf.png (from extension "pdf", or from the subtype of the mimetype "application/pdf")

video.gif (from the mediatype of the mimetype "video/avi")

wav.ico (from the subtype of the mimetype "audio/x-wav")

You can download a big set of file icons from http://www.splitbrain.org/projects/file_icons.

Retrieving a file icon:
Oftenly, you will have to show a nice file icon to let people know easily what type of file is being presented. To use it, simply use the method "icon" of this helper:

<?php
// will create an icon by passing a mimetype:
$mimetype = "image/jpg";
$this->Icon->get( $mimetype );

// will create an icon by passing an extension:
$extension = "pdf";
$this->Icon->get( $extension );

// will create an icon by mimetype and use an extension as a fallback 
// if no icon could be determined by the mimetype:
$mimetype = "application/octet-stream";
$extension = "avi";
$this->Icon->get( $mimetype, $extension );
?>
To use a different fileicon-set, you can define the path to it. It must be relative to Cake's images folder.

<?php
$this->Icon->setFileiconsPath( "other_fileicons" );  // lets IconHelper use app/webroot/img/other_fileicons for icon retrieval
?>