Hey, I am kinda new to this whole "contributing thing". So, I have no idea if this is the best way to contribute, yet, I was struggling with a similar issue with SVG instead of ICO files and wanted to share it here.

1. I added SVG support and wanted to upload a .svg file as the Site Icon (Appearance >> Customize >> Site Identity).
2. I uploaded the .svg first. That went well since I had enabled SVG support inside functions.php.
3. I wanted to select the uploaded .svg as a Site Icon. I selected the image, but WP asked me to crop the image. I could not do that since WP is not set up to crop vector images. I clicked on "Crop" anyway and WP gave me an error, that the image can't be cropped. Suprise, surprise...

To fix the issue, I added the following code in the same file /wp-admin/includes/ajax-actions.php

function wp_ajax_crop_image() {

//...rest of the code

if ( empty( $attachment_id ) || ! current_user_can( 'edit_post', $attachment_id ) ) {
  wp_send_json_error();
}

// THE CODE I ADDED BELLOW

// Check if the attachment is an SVG file
    $mime_type = get_post_mime_type( $attachment_id );
    if ( 'image/svg+xml' === $mime_type ) {
        // For SVG files, send the original image data without cropping
        wp_send_json_success( wp_prepare_attachment_for_js( $attachment_id ) );
    }

// THE CODE I ADDED ABOVE

$context = str_replace( '_', '-', $_POST['context'] );
$data    = array_map( 'absint', $_POST['cropDetails'] );
$cropped = wp_crop_image( $attachment_id, $data['x1'], $data['y1'], $data['width'], $data['height'], $data['dst_width'], $data['dst_height'] );

//...rest of the code
}

What it does, is it checks if the image that is going to be cropped is an SVG, if it is and you click on "Crop" it does not bother to crop it at all and returns the image as it is.

This way I was able to designate the SVG file I uploaded as a Site Icon.

I am not saying that this is a perfect solution, but I do not see the point in cropping an SVG at all. So it works for me...

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