WP Attachment Multiple Origin support plugin. Helps to override Sage`s `UPLOADS_URL` filter with proper origin.

.env

# For production no filters will be applied
WP_ENV=development
WP_HOME=http://example.com
WP_SITEURL=${WP_HOME}/wp

# Should be defined for staging and development, has no effect for production
UPLOADS_URL='http://example.com/app/uploads'

# Should be defined for development, maybe skipped for staging if Bedrock used
UPLOADS_URL_STAGING='//stage.example.com/app/uploads'

# Maybe skipped for development if Bedrock used
UPLOADS_URL_DEVELOPMENT='//example.test/app/uploads'

attachments-multiple-origins.php

<?php
/**
 * @wordpress-plugin
 * Plugin Name: Attachments Multiple Origins
 * Description: Filters allowing to load attachments with different origin. Useful with "sage" `UPLOADS_URL` constant. Optionally maybe defined env-based constants: `UPLOADS_URL_DEVELOPMENT='http://dev/app/uploads'; UPLOADS_URL_STAGING='http://staging/app/uploads'`
 * Author:      Alex Batalov
 * License:     GPL-2.0+
 */

namespace WPAttachmentsMultipleOrigins;

function get_uploads_origin_id() {
    if (!defined('WP_ENV')) {
        return null;
    }

    return WP_ENV . '-' . wp_hash(WP_ENV);
}

/**
 * Replace UPLOADS_URL by origin based url
 *
 * @param string $attachment_url
 * @param int $post_id
 * @return string
 */
function adjust_attachment_url_origin($attachment_url, $post_id) {
    if (!defined('UPLOADS_URL') or !defined('WP_ENV')) {
        return $attachment_url;
    }

    if (!$attachment_meta = wp_get_attachment_metadata($post_id)) {
        return $attachment_url;
    }

    if (!$origin_meta = $attachment_meta['origin'] ?? false) {
        return $attachment_url;
    }

    [$origin_env] = $origin = explode('-', $origin_meta);

    // if origin_env and current env are same but different hash, possible from one of developer db, imported into stage
    if ($origin_env === WP_ENV and $origin_meta !== get_uploads_origin_id()) {
        return $attachment_url;
    }

    $uploads_url_constant_name = 'UPLOADS_URL_' . mb_strtoupper($origin_env);

    // keep ability to modify constant name
    $uploads_url_constant_name = apply_filters('uploads_origin/constant_name', $uploads_url_constant_name, $attachment_url, $post_id, $origin);

    if (defined($uploads_url_constant_name)) {
        if (!$uploads_url = constant($uploads_url_constant_name)) {
            return $attachment_url;
        }
    }
    else {
        // this is stage possible and self uploads env not defined
        if ($origin_env === WP_ENV and defined('WP_HOME')) {
            // Bedrock hierarchy used as default
            $uploads_url = untrailingslashit(WP_HOME) . '/app/uploads';
        }
        else {
            return $attachment_url;
        }
    }

    $uploads_url = apply_filters('uploads_origin/url', $uploads_url, $attachment_url, $post_id, $origin);

    $attachment_url = $uploads_url . mb_substr($attachment_url, mb_strlen(UPLOADS_URL));

    return $attachment_url;
}

function add_upload_origin_filters() {
    if (!defined('UPLOADS_URL')) {
        return;
    }

    /**
     * Update images urls
     */
    add_filter('pre_option_upload_url_path', function () {
        return UPLOADS_URL;
    });

    add_filter('wp_get_attachment_url', function ($attachment_url, $post_id) {
        return adjust_attachment_url_origin($attachment_url, $post_id);
    }, 10, 2);

    add_filter('wp_calculate_image_srcset', function ($sources, $size_array, $image_src, $image_meta, $attachment_id) {
        foreach ($sources as &$source) {
            $source['url'] = adjust_attachment_url_origin($source['url'], $attachment_id);
        }

        return $sources;
    }, 10, 5);

    add_filter('wp_update_attachment_metadata', function ($data) {
        if (!defined('UPLOADS_URL') or !defined('WP_HOME')) {
            return $data;
        }

        // If UPLOADS_URL seems as normal wp uploads url don't need to keep origin id DB.
        // At this case we probably have no `production` (useful for stage env)
        // or probably have no `stage` (useful for development env when one developer started project)
        if (mb_strpos(UPLOADS_URL, WP_HOME) === 0) {
            return $data;
        }

        // Store in format: `environment`-`hash`
        // It allows to prevent multiple wp installs(dev) inconsistency
        $data['origin'] = get_uploads_origin_id();

        return $data;
    });
}

if (defined('WP_ENV') and WP_ENV !== 'production' and defined('UPLOADS_URL')) {
    add_upload_origin_filters();
}

wp-config.php

<?php
/**
 * In case WodPress is installed without using Bedrock, the following constants must be added
 *
 **/

// Environtment constant
define('WP_ENV', 'development');
// `UPLOADS_URL` should be defined for staging and development, has no effect for production
define('UPLOADS_URL', 'http://example.com/app/uploads');
// Environtment specific uploads URL
define('UPLOADS_URL_DEVELOPMENT', '//example.test/app/uploads');