Use modified ActionDispatch::Static to let uploads go through to routes.

config/initializers/static_files.rb

@@ -0,0 +1,13 @@
+begin
+  app = Rails.application
+
+  app.config.middleware.swap(
+    ActionDispatch::Static, 
+    Gitlab::Middleware::Static, 
+    app.paths["public"].first, 
+    app.config.static_cache_control
+  )
+rescue
+  # If ActionDispatch::Static wasn't loaded onto the stack (like in production), 
+  # an exception is raised.
+end

lib/gitlab/middleware/static.rb

@@ -0,0 +1,13 @@
+module Gitlab
+  module Middleware
+    class Static < ActionDispatch::Static
+      UPLOADS_REGEX = /\A\/uploads(\/|\z)/.freeze
+
+      def call(env)
+        return @app.call(env) if env['PATH_INFO'] =~ UPLOADS_REGEX
+
+        super
+      end
+    end
+  end
+end