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.

db/dev_seeds/budgets.rb

@@ -15,14 +15,12 @@ end
 
 def add_image_to(imageable)
   # imageable should respond to #title & #author
-  File.open(INVESTMENT_IMAGE_FILES.sample) do |file|
-    imageable.image = Image.create!({
-      imageable: imageable,
-      title: imageable.title,
-      attachment: file,
-      user: imageable.author
-    })
-  end
+  imageable.image = Image.create!({
+    imageable: imageable,
+    title: imageable.title,
+    attachment: Rack::Test::UploadedFile.new(INVESTMENT_IMAGE_FILES.sample),
+    user: imageable.author
+  })
   imageable.save!
 end
 

db/dev_seeds/proposals.rb

@@ -12,14 +12,12 @@ end
 
 def add_image_to(imageable)
   # imageable should respond to #title & #author
-  File.open(IMAGE_FILES.sample) do |file|
-    imageable.image = Image.create!({
-      imageable: imageable,
-      title: imageable.title,
-      attachment: file,
-      user: imageable.author
-    })
-  end
+  imageable.image = Image.create!({
+    imageable: imageable,
+    title: imageable.title,
+    attachment: Rack::Test::UploadedFile.new(IMAGE_FILES.sample),
+    user: imageable.author
+  })
   imageable.save!
 end
 

db/dev_seeds/widgets.rb

@@ -1,7 +1,7 @@
 section "Creating header and cards for the homepage" do
   def create_image_attachment(type)
     {
-      attachment: File.new(Rails.root.join("db/dev_seeds/images/#{type}_background.jpg")),
+      attachment: Rack::Test::UploadedFile.new("db/dev_seeds/images/#{type}_background.jpg"),
       title: "#{type}_background.jpg",
       user: User.first
     }