File placeholders not generated in return value of of $block->display()

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I'm trying to render blocks from a tool script for use on another site; everything works well, except file placeholders in the content are not getting replaced like they normally do, what is missing?

Here's an example of an image that would be rendered as part of the content, missing the actual src:
<img src="{CCM:FID_407}" />


Here's some sample code being used to render the blocks

<?php
$blocks = $someGlobalArea->getAreaBlocksArray($somePage);
foreach($blocks as $block) {
    $block->display('templates/some_template');
}