.rb

# controller
def create
  @errors = []
  
  params[:files].each |file|
    document = Document.new(file:, ...)
    if document.save
      # handle success
    else
      # ...
      @errors = document.errors
    end
  end
  redirect_to_or_back root_path
end

# view
%h1= t('.title')
= form_with scope: documents, url: my_path do |f|
  if @errors && @errors.any?
    - @errors.full_messages.each do |message|
      %p= message
  %div.field
   ...