Media: Account for Windows when normalizing file paths.

Previously, Windows paths in the `path_is_absolute` function resulted in incorrect URIs. This patch adjusts for forward slashes and adds tests for the `get_attached_file` function.
Props Whissi, SergeyBiryukov, desrosj, stevenlinx, birgire, davidbaumwald, costdev, peterwilsoncc, audrasjb, hellofromTonya, johnbillion.
Fixes #36308.



git-svn-id: https://develop.svn.wordpress.org/trunk@53934 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -2083,6 +2083,7 @@ function wp_mkdir_p( $target ) {
  * For example, '/foo/bar', or 'c:\windows'.
  *
  * @since 2.5.0
+ * @since 6.1.0 Allows normalized Windows paths (forward slashes).
  *
  * @param string $path File path.
  * @return bool True if path is absolute, false is not absolute.
@@ -2113,6 +2114,11 @@ function path_is_absolute( $path ) {
 		return true;
 	}
 
+	// Normalized Windows paths for local filesystem and network shares (forward slashes).
+	if ( preg_match( '#(^[a-zA-Z]+:/|^//[\w!@\#\$%\^\(\)\-\'{}\.~]{1,15})#', $path ) ) {
+		return true;
+	}
+
 	// A path starting with / or \ is absolute; anything else is relative.
 	return ( '/' === $path[0] || '\\' === $path[0] );
 }