Accessing a user file through an injected script (Safari App Extension macOS)

I am trying to use an otf file that the user selects from their computer in an injected script from my Safari App Extension (macOS), but currently, I can not get Safari to load the file due to permission issues.

This is what I am currently doing:

First I allow the user to select a file (code below):

var newFileURL = "";
let openPanel = NSOpenPanel()
openPanel.allowedFileTypes = ["otf"]
openPanel.allowsMultipleSelection = false
openPanel.canChooseDirectories = false
openPanel.canChooseFiles = true
openPanel.runModal()
        
newFileURL = openPanel.url?.absoluteString ?? ""
NSLog(newFileURL)

Next I send the variable ‘newFileUrl’ as a message to the default script.js and I handle the message with this javascript code:

function handleMessage(event) {
var css = "@font-face {font-family: userFont; src: url('file:///" + event.name + "');}",
head = document.head || document.getElementsByTagName('head')[0],
style = document.createElement('style');

    console.log(event.name);
    
    head.appendChild(style);
    
    style.type = 'text/css';
    if (style.styleSheet){
        style.styleSheet.cssText = css;
    } else {
        style.appendChild(document.createTextNode(css));
    }

}

When I do this I get the an error in the safari developer console, “Not allowed to load local resource: file:///Users/macuser/Downloads/file.otf”

I have allowed read/write permissions for user files for my extension so I am not sure how I would go about allowing user files for the injected script.

Thanks!