Fix invisible error message in attachments

In commit cc6f939 we made the images and documents file inputs
invisible (instead of using `display: none`) in order to make it
possible to attach images and documents using the keyboard.

However, since the error messages associated to these inputs has the
same HTML class as the inputs, we were also hiding them (the `display:
none` didn't affect the error messages because they've also got the
`is-visible` class).

Using the `[type=file]` selector we make it more explicit that we only
want to style these inputs.

I'm not adding a test for this scenario because technically the text is
there and I'm not sure how to test for the presence of invisible
elements.

app/assets/stylesheets/mixins/uploads.scss

@@ -28,8 +28,7 @@
 
     .attachment-errors {
 
-      > .js-image-attachment,
-      > .js-document-attachment {
+      > [type=file] {
         @include element-invisible;
 
         ~ .error {