Media: Implement fallback to full-size thumbnails for PDFs in Media L…

…ibrary.

When medium-size thumbnails are disabled, PDF previews in the Media Library grid view now fall back to displaying full-size thumbnails instead of a generic document icon.

Props daleharrison, kushang78, mukesh27, sudipatel007, mai21, antpb.
Fixes #39630.


git-svn-id: https://develop.svn.wordpress.org/trunk@58947 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media-template.php

@@ -605,8 +605,12 @@ function wp_print_media_templates() {
 					<div class="centered">
 						<# if ( data.image && data.image.src && data.image.src !== data.icon ) { #>
 							<img src="{{ data.image.src }}" class="thumbnail" draggable="false" alt="" />
-						<# } else if ( data.sizes && data.sizes.medium ) { #>
-							<img src="{{ data.sizes.medium.url }}" class="thumbnail" draggable="false" alt="" />
+						<# } else if ( data.sizes ) { 
+								if ( data.sizes.medium ) { #>
+									<img src="{{ data.sizes.medium.url }}" class="thumbnail" draggable="false" alt="" />
+								<# } else { #>
+									<img src="{{ data.sizes.full.url }}" class="thumbnail" draggable="false" alt="" />
+								<# } #>
 						<# } else { #>
 							<img src="{{ data.icon }}" class="icon" draggable="false" alt="" />
 						<# } #>