Tests: Improve Windows compatibility (#249)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: swissspidy

features/media-import.feature

@@ -325,7 +325,8 @@ Feature: Manage WordPress attachments
       my-image.png.png
       """
   Scenario: Fail to import from STDIN when no input provided
-    When I try `wp media import - </dev/null`
+    When I run `php -r "file_put_contents('empty.txt', '');"`
+    And I try `wp media import - < empty.txt`
     Then STDERR should contain:
       """
       Warning: Unable to import file from STDIN. Reason: No input provided.

src/Media_Command.php

@@ -571,7 +571,7 @@ public function import( $args, $assoc_args = array() ) {
 				}
 
 				// Determine file extension from content
-				$mimetype = mime_content_type( $tempfile );
+				$mimetype = function_exists( 'mime_content_type' ) ? mime_content_type( $tempfile ) : '';
 
 				// Map MIME type to extension
 				$ext = '';