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

tests/phpunit/tests/functions.php

@@ -105,6 +105,13 @@ class Tests_Functions extends WP_UnitTestCase {
 			'C:\\',
 			'C:\\WINDOWS',
 			'\\\\sambashare\\foo',
+			'c:/',
+			'c://',
+			'//',
+			'c:/FOO',
+			'//FOO',
+			'C:/WWW/Sites/demo/htdocs/wordpress/wp-content/uploads/2016/03/example.jpg',
+			'//ComputerName/ShareName/SubfolderName/example.txt',
 		);
 		foreach ( $absolute_paths as $path ) {
 			$this->assertTrue( path_is_absolute( $path ), "path_is_absolute('$path') should return true" );
@@ -119,10 +126,14 @@ class Tests_Functions extends WP_UnitTestCase {
 			'../foo',
 			'../',
 			'../foo.bar',
+			'foo.bar',
 			'foo/bar',
 			'foo',
 			'FOO',
 			'..\\WINDOWS',
+			'..//WINDOWS',
+			'c:',
+			'C:',
 		);
 		foreach ( $relative_paths as $path ) {
 			$this->assertFalse( path_is_absolute( $path ), "path_is_absolute('$path') should return false" );