Enable the new multipart handler by default

Update the related documentation

config/feature_flags/development/upload_middleware_jwt_params_handler.yml

@@ -4,4 +4,4 @@ introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/33277
 rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/233895
 group: group::package
 type: development
-default_enabled: false
+default_enabled: true

doc/administration/uploads.md

@@ -2,6 +2,35 @@
 
 Uploads represent all user data that may be sent to GitLab as a single file. As an example, avatars and notes' attachments are uploads. Uploads are integral to GitLab functionality, and therefore cannot be disabled.
 
+## Upload parameters
+
+> - [Changed](https://gitlab.com/gitlab-org/gitlab/-/issues/214785) in GitLab 13.5.
+> - It's [deployed behind a feature flag](../user/feature_flags.md), enabled by default.
+> - It's enabled on GitLab.com.
+> - It's recommended for production use.
+> - For GitLab self-managed instances, GitLab administrators can opt to disable it. **(CORE ONLY)**
+
+In 13.5 and later, upload parameters are passed [between Workhorse and GitLab Rails](../development/architecture.md#simplified-component-overview) differently than they
+were before.
+
+This change is deployed behind a feature flag that is **enabled by default**.
+
+If you experience any issues with upload,
+[GitLab administrators with access to the GitLab Rails console](./feature_flags.md)
+can opt to disable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:upload_middleware_jwt_params_handler)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:upload_middleware_jwt_params_handler)
+```
+
 ## Using local storage
 
 NOTE: **Note:**

lib/gitlab/middleware/multipart.rb

@@ -229,7 +229,7 @@ module Gitlab
       private
 
       def handler_class
-        if Feature.enabled?(:upload_middleware_jwt_params_handler)
+        if Feature.enabled?(:upload_middleware_jwt_params_handler, default_enabled: true)
           ::Gitlab::Middleware::Multipart::HandlerForJWTParams
         else
           ::Gitlab::Middleware::Multipart::Handler

spec/support/helpers/multipart_helpers.rb

@@ -31,7 +31,7 @@ module MultipartHelpers
       raise ArgumentError, "can't handle #{mode} mode"
     end
 
-    return result if ::Feature.disabled?(:upload_middleware_jwt_params_handler)
+    return result if ::Feature.disabled?(:upload_middleware_jwt_params_handler, default_enabled: true)
 
     # the HandlerForJWTParams expects a jwt token with the upload parameters
     # *without* the "#{key}." prefix