Workaround for accepting PDF uploads in backend Magento after SUPEE-9767 v1/v2

config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Vendor_ExtendedValidator>
            <version>0.1.0</version>
        </Vendor_ExtendedValidator>
    </modules>
    <global>
        <models>
            <vendor_extendedvalidator>
                <class>Vendor_ExtendedValidator_Model</class>
            </vendor_extendedvalidator>
            <core>
                <rewrite>
                    <file_validator_image>Vendor_ExtendedValidator_Model_File_Validator_Image</file_validator_image>
                </rewrite>
            </core>
        </models>
    </global>
</config>

Image.php

<?php
/**
 * Created by PhpStorm.
 * User: sander
 * Date: 25-7-17
 * Time: 10:16
 */
class Vendor_ExtendedValidator_Model_File_Validator_Image extends Mage_Core_Model_File_Validator_Image
{
    /**
     * Allowed Mime Content Types
     * @return array
     */
    protected function getAllowedMimeContentTypes()
    {
        return ['application/pdf'];
    }


    /**
     * If Mime Content Type is allowed return null, else validate with parent
     *
     * @param string $filePath
     * @return null
     */
    public function validate($filePath)
    {
        $allowedMimeContentTypes = $this->getAllowedMimeContentTypes();

        if (in_array(mime_content_type($filePath), $allowedMimeContentTypes)) {
            return null;
        }

        parent::validate($filePath);
    }
}