Check if a local file exists in the WordPress media library and if it doesn't exist, replace the URL with a different URL. Helpful when working with a local site that doesn't have all of the media downloaded as the production site. See https://localwp.com/community/t/proxying-requests-to-wp-content-uploads-to-a-production-site/15701

use-remote-media.php

<?php
// Put this in wp-config.php and replace https://example.com/ with the URL of the production site.
define( 'RH_USE_REMOTE_MEDIA_URL', 'https://example.com/' );

// Put the rest of this in functions.php or a custom plugin or somewhere else.
if ( defined( 'RH_USE_REMOTE_MEDIA_URL' ) && ! empty( RH_USE_REMOTE_MEDIA_URL ) ) {
	add_filter( 'wp_get_attachment_image_src', 'filter_wp_get_attachment_image_src' );
	add_filter( 'wp_calculate_image_srcset', 'filter_wp_calculate_image_srcset' );
	add_filter( 'wp_get_attachment_url', 'filter_wp_get_attachment_url' );
}

function filter_wp_get_attachment_image_src( $image = array() ) {
	if ( ! is_array( $image ) || empty( $image ) ) {
		return $image;
	}
	$wp_upload_dir = wp_upload_dir();
	$base_dir      = $wp_upload_dir['basedir'];
	$base_url      = $wp_upload_dir['baseurl'];
	$absolute_path = str_replace( $base_url, $base_dir, $image[0] );
	if ( file_exists( $absolute_path ) ) {
		return $image;
	}
	$find     = get_site_url();
	$replace  = RH_USE_REMOTE_MEDIA_URL;
	$image[0] = str_replace( $find, $replace, $image[0] );
	return $image;
}

function filter_wp_calculate_image_srcset( $src = array() ) {
	if ( is_array( $src ) && ! is_admin() ) {
		$wp_upload_dir = wp_upload_dir();
		$base_dir      = $wp_upload_dir['basedir'];
		$base_url      = $wp_upload_dir['baseurl'];
		$find          = get_site_url();
		$replace       = RH_USE_REMOTE_MEDIA_URL;
		foreach ( $src as $key => $val ) {
			$absolute_path = str_replace( $base_url, $base_dir, $val['url'] );
			if ( ! file_exists( $absolute_path ) ) {
				$val['url']  = str_replace( $find, $replace, $val['url'] );
				$src[ $key ] = $val;
			}
		}
	}
	return $src;
}

function filter_wp_get_attachment_url( $url = '' ) {
	if ( is_admin() ) {
		return $url;
	}

	$wp_upload_dir = wp_upload_dir();
	$base_dir      = $wp_upload_dir['basedir'];
	$base_url      = $wp_upload_dir['baseurl'];
	$find          = get_site_url();
	$replace       = RH_USE_REMOTE_MEDIA_URL;
	$absolute_path = str_replace( $base_url, $base_dir, $url );
	if ( ! file_exists( $absolute_path ) ) {
		$url = str_replace( $find, $replace, $url );
	}
	return $url;
}

@bacoords if you have a Composer based site setup I took this work and created a MU Plugin that can be installed as a dev dependency on a site. I have addressed that trailing slash issue in my version of the code.

https://packagist.org/packages/ndigitals/wp-local-media-proxy

Also if you're running WordPress on an Apache server you can accomplish the same thing by dropping a .htaccess file into /wp-content/uploads/ with the following contents:

RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule (.*) https://example.com/wp-content/uploads/$1 [L]

Replace https://example.com with the production URL of your site to redirect media requests to.