Media: Clearly indicate that PDF thumbnails are just document previews.

This helps affirm that your upload is fine, and has not been reduced down to the first page or changed into an image. This applies to the attachment details modal in the media library, where media is also now centered in the preview area.

props folletto, helen.
fixes #38717.

Built from https://develop.svn.wordpress.org/trunk@39213


git-svn-id: http://core.svn.wordpress.org/trunk@39153 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/media-rtl.css

@@ -769,13 +769,17 @@ border color while dragging a file over the uploader drop area */
 
 .edit-attachment-frame .attachment-media-view .details-image {
 	display: block;
-	margin-bottom: 16px;
+	margin: 0 auto 16px;
 	max-width: 100%;
 	max-height: 90%;
 	max-height: -webkit-calc( 100% - 42px );
 	max-height: calc( 100% - 42px ); /* leave space for actions underneath */
 }
 
+.edit-attachment-frame .attachment-media-view .attachment-actions {
+	text-align: center;
+}
+
 .edit-attachment-frame .wp-media-wrapper {
 	margin-bottom: 12px;
 }

wp-admin/css/media.css

@@ -769,13 +769,17 @@ border color while dragging a file over the uploader drop area */
 
 .edit-attachment-frame .attachment-media-view .details-image {
 	display: block;
-	margin-bottom: 16px;
+	margin: 0 auto 16px;
 	max-width: 100%;
 	max-height: 90%;
 	max-height: -webkit-calc( 100% - 42px );
 	max-height: calc( 100% - 42px ); /* leave space for actions underneath */
 }
 
+.edit-attachment-frame .attachment-media-view .attachment-actions {
+	text-align: center;
+}
+
 .edit-attachment-frame .wp-media-wrapper {
 	margin-bottom: 12px;
 }