Remove hack to allow IO files in Active Storage

We were using this hack in order to allow `File.new` attachments in
tests files. However, we can use the `fixture_file_upload` helper
instead.

Just like it happened with `file_fixture`, this helper method doesn't
work in fixtures, so in this case we're using `Rack::Test::UploadedFile`
instead.

app/models/concerns/has_attachment.rb

@@ -6,9 +6,7 @@ module HasAttachment
       has_one_attached attribute
 
       define_method :"#{attribute}=" do |file|
-        if file.is_a?(IO)
-          send(attribute).attach(io: file, filename: File.basename(file.path))
-        elsif file.nil?
+        if file.nil?
           send(attribute).detach
         else
           send(attribute).attach(file)