Upload file

<% model = (param.respond_to? :object) ? param.object : param %>

<%   
   uploader_name = model.class.try(:uploaders).try(:keys).try(:first).try(:to_sym) # suppose the model has only one uploader
   uploader = model.send(uploader_name)     
   url = ((uploader._storage.to_s == "CarrierWave::Storage::File") ? request.base_url.to_s : "") + uploader.url.to_s
%>

<div class="field">
  <%= label_tag uploader_name.to_s.capitalize + ":"%><br>
  <% if uploader.file %>   
    <%= image_tag url  %><br>
    <div>Image file name: <%= uploader.file.filename  %></div><br><br>
  <% end %>
</div>

<% if param.respond_to? :object %>

  <% form = param %>
  
  <div class="field">
    <%= form.file_field uploader_name %>
  </div><br>

  <div class="field">
    <label>
      <%= form.check_box "remove_#{uploader_name}" %>
      Remove file
    </label>
  </div><br><br>

<% end %>

<%# console %>

USAGE EXAMPLE:

How to call this partial:

class User < ApplicationRecord
  mount_uploader :avatar, AvatarUploader
end

Show action:
<%#= render 'layouts/uploaded_file', param: @user %>

Edit action:
<%#= render 'layouts/uploaded_file', param: form %>