Media: Fix exif_imagetype check in wp_get_image_mime

This is a follow up to [39831].

Merges [39850] to the 4.1 branch.

Built from https://develop.svn.wordpress.org/branches/4.1@39857


git-svn-id: http://core.svn.wordpress.org/branches/4.1@39794 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/functions.php

@@ -2179,7 +2179,7 @@ function wp_get_image_mime( $file ) {
 	 * we assume the file could not be validated.
 	 */
 	try {
-		if ( ! is_callable( 'exif_imagetype' ) ) {
+		if ( is_callable( 'exif_imagetype' ) ) {
 			$mime = image_type_to_mime_type( exif_imagetype( $file ) );
 		} elseif ( function_exists( 'getimagesize' ) ) {
 			$imagesize = getimagesize( $file );