Fix tiresome headers already sent bug in Magento 1.x

ImageController.php

<?php
    /**
     * Generate image thumbnail on the fly
     */
    public function thumbnailAction()
    {
        $file = $this->getRequest()->getParam('file');
        $file = Mage::helper('cms/wysiwyg_images')->idDecode($file);
        $thumb = $this->getStorage()->resizeOnTheFly($file);
        if ($thumb !== false) {
            $image = Varien_Image_Adapter::factory('GD2');
            $image->open($thumb);
            ob_start();
            $image->display();
            $imageData = ob_get_contents();
            ob_end_clean();
            $mimeType = image_type_to_mime_type($image->getMimeType());
            $this->getResponse()->setHeader('Content-Type', $mimeType, true);
            $this->getResponse()->setBody($imageData);
        } else {
            // todo: genearte some placeholder
        }
    }

Basically the problem is in $image->display() an explict header('Content-Type', $mimeType) is set. It actually is even buggier still, as in the case of a PNG, it sets the content-type to 3 (gd's PNG filetype code).

This fix works around the problem by capturing the image data using output buffering, then uses the fact that getMimeType() returns a file type id rather than actual mimetype to query the correct mimetype from the gd type database.

There's more elegant solutions to this, but I wanted to change the absolute minimum of core magento code and have those changes as localized as possible.