fix(file_input): append [] to the name for multiple-file inputs

CHANGELOG.md

@@ -7,6 +7,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- **`FileInput` multiple uploads submit an array** (Rails `file_field` parity):
+  with `multiple:` set, the input name now gets `[]` appended (unless already
+  present), for the standalone component, the `Plain` variant, and the form
+  builder path. Without it the browser collapsed a multi-file selection into
+  ONE scalar param, which a host app's `params.expect(attr: [])` silently
+  discarded — uploads no-opped with a success response.
+
 ### Added
 
 - **`checkbox_group` — batched checkbox group for array-valued fields** (the

lib/forms/file_input.rb

@@ -8,7 +8,10 @@ class FileInput < Phlex::HTML
     def initialize(*modifiers, name: nil, id: nil, multiple: false, accept: nil,
                    error: false, disabled: false, required: false, full_width: true, **attributes)
       @modifiers = normalize_modifiers(modifiers)
-      @name = name
+      # Rails file_field parity: a multiple input needs an array param name, or
+      # the browser collapses the selection into one scalar file — which a host
+      # app's `params.expect(attr: [])` then silently discards.
+      @name = multiple && name && !name.to_s.end_with?("[]") ? "#{name}[]" : name
       @id = id
       @multiple = multiple
       @accept = accept

spec/forms/file_input_spec.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+# Rails' file_field appends [] to the input name when multiple is set; without
+# it the browser collapses a multi-file selection into ONE scalar param, which
+# a host app's `params.expect(attr: [])` silently discards — uploads no-op
+# with a success response (cosmos production, Aug 2026).
+describe Forms::FileInput do
+  it "keeps the plain name for single-file inputs" do
+    output = render_component(described_class.new(name: "user[avatar]"))
+
+    expect(output).to include('name="user[avatar]"')
+    expect(output).not_to include("multiple")
+  end
+
+  it "appends [] to the name when multiple (Rails file_field parity)" do
+    output = render_component(described_class.new(name: "retreat[gallery]", multiple: true))
+
+    expect(output).to include('name="retreat[gallery][]"')
+    expect(output).to include("multiple")
+  end
+
+  it "does not double-append when the caller already passed []" do
+    output = render_component(described_class.new(name: "retreat[gallery][]", multiple: true))
+
+    expect(output).to include('name="retreat[gallery][]"')
+    expect(output).not_to include("[][]")
+  end
+
+  it "appends [] through the form builder too" do
+    user = build_model(:user, name: "Ada")
+    output = PhlexHelpers::FormContext.new(
+      model: user, form_args: {},
+      form_block: ->(f) { f.FileInput(:name, multiple: true) }
+    ).call
+
+    expect(output).to include('name="user[name][]"')
+  end
+
+  it "applies the same normalization to the Plain variant" do
+    output = render_component(Forms::Plain::FileInput.new(name: "retreat[gallery]", multiple: true))
+
+    expect(output).to include('name="retreat[gallery][]"')
+  end
+end