Validator Zend_Validate_File_IsImage doesn't work correct

Question

I try to use the next validator of ZF:

$isImageValidator = new Zend_Validate_File_IsImage(array('headerCheck' => true));

(I don'n want to use fileInfo or mimeMagic)

But even when image is chosen, validator returns next:

File 'image.png' is no image, 'application/octet-stream' detected

(When i look through the $_FILES global array it detects 'image/png') Why???

Help me please...