The Zend Validator error messages are not the most helpful. Considor the below code for creating a file input:

$this->addElement('file', 'signature', array(
    'validators' => array(
        array('Size', false, 20480), //20k
        array('Extension', false, 'png'),
    ),
    'required' => false,
    'label' => 'Signature',
    'destination' => SIG_PATH,
));

For instance, on the above file input, if you attempt to upload something that is not a PNG file (i.e. photo.JPG, the message will be:

File 'photo.JPG' has a false extension

That’s not very user friendly as it doesn’t give the user any indication of what is an “allowed” file type.

So, to combat this, create separate Zend Validators and attach them to the Form Elements separately:

$this->addElement('file', 'signature', array(
    'required' => false,
    'label' => 'Signature',
    'destination' => SIG_PATH,
    'maxfilesize' => 20480,
));

$fileSize = new Zend_Validate_File_Size(20480);
$fileSize->setMessage('Please only upload a picture with a maximum 20kb file size');

$fileType = new Zend_Validate_File_Extension('png');
$fileType->setMessage('Only .png files are acceptable for signatures.');

$sig = $this->getElement('signature');
$sig->addValidators(array($fileSize, $fileType));

This way you have total control over the messages sent to the user. And after all, user experience is key these days.