By default, LocomotiveCMS will not let you upload any file type of theme assets.

Just only the default ones:

jpg jpeg gif png css js swf flv eot svg ttf woff otf ico htc

For instance, PDF files are not allowed. To change that, you need to override one of the methods of the Locomotive::ThemeAssetUploader class.

First, create or modify the config/initializers/engine_overrides.rb file by adding the following piece of code:

Locomotive::ThemeAssetUploader.class_eval do
  def extension_white_list
    %w(jpg jpeg gif png css js swf flv eot svg ttf woff otf ico htc pdf)
  end
end

Restart your engine, it is done.