Custom "Uploads" Dir: "Edit Image" in Media Library broken

Question

Directory Structure

I have customized my (local) install (dev box). The current directory structure is:

 ./root
  |- wp (WordPress Core root)
    \
    |- wp-config.php
  |- wp-content (Content and Vendor root)
    \
    |- mu-plugins (...)
    |- themes (...)
    |- plugins (...)
    |- uploads (Custom uploads dir)
    |- vendor (Composer installed libraries)
  |- index.php (loads ~/wp/index.php > require( dirname(__FILE__).'/wp/wp-blog-header.php' ); )

Config

In my wp-config.php, I have the following setup:

define( 'WP_SITEURL',      'http://example.dev/wp' );
define( 'WP_HOME',         WP_SITEURL );
define( 'WP_CONTENT_DIR',  './../../wp-content' );
define( 'WP_CONTENT_URL',  'http://example.dev/wp-content' );
define( 'WP_PLUGIN_DIR',   '../../wp-content/plugins' );
define( 'WP_PLUGIN_URL',   WP_CONTENT_URL.'/plugins' );
define( 'WPMU_PLUGIN_DIR', '../../wp-content/mu-plugins' );
define( 'WPMU_PLUGIN_URL', WP_CONTENT_URL.'/mu-plugins' );

It is a single site install and none of the following constants is defined (and therefore falls back to default): UPLOADBLOGSDIR, UPLOADS, BLOGUPLOADDIR.

Filters

As @Sven (Hofmann) asked me in chat: The only plugin running on the *_url filters is one mu-plugin that sets up the new themes directory:

add_filter( 'theme_root_uri', 'switch_theme_root_local' );
add_filter( 'theme_root',     'switch_theme_root_local' );

As you can see, there's nothing that can conflict with internal URls.

Problem

Step 1) When I go to the "Media Library" and click/open an attachment, the modal opens like normal and displays the media file (or in the problematic case: the image).

Step 2) Hit the "Edit Image" Link above and the following screen (modal replacement) appears. Note the broken image. I opened Chromes dev tools to show the el and attributes/path.

I have no idea how to debug this and/or where the broken links origin could be. I assume that this is burried somewhere in WordPress least documented parts: JavaScript templates for the media library.

Answer 1

The uploads dir has its own define in wp-config:

define( 'UPLOADS', WP_CONTENT_URL.'/customuploads' );