[FileUtils] Fix getExtension() treatment of path separators.

This CL fixes how org.chromium.base.FileUtils.getExtension() treats
periods in paths. Also add test for getExtension().

Bug: 1067272, 1066730
Change-Id: I07d85727da90d508d69aa8a6ef026aabbdf19aed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134577
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756041}

base/android/java/src/org/chromium/base/FileUtils.java

@@ -167,13 +167,14 @@ public class FileUtils {
 
     /**
      * Returns the file extension, or an empty string if none.
-     * @param file Name of the file, with or without the full path.
+     * @param file Name of the file, with or without the full path (Unix style).
      * @return empty string if no extension, extension otherwise.
      */
     public static String getExtension(String file) {
-        int index = file.lastIndexOf('.');
-        if (index == -1) return "";
-        return file.substring(index + 1).toLowerCase(Locale.US);
+        int lastSep = file.lastIndexOf('/');
+        int lastDot = file.lastIndexOf('.');
+        if (lastSep >= lastDot) return ""; // Subsumes |lastDot == -1|.
+        return file.substring(lastDot + 1).toLowerCase(Locale.US);
     }
 
     /** Queries and decodes bitmap from content provider. */

base/android/junit/src/org/chromium/base/FileUtilsTest.java

@@ -212,6 +212,26 @@ public class FileUtilsTest {
     // TOOD(huangs): Implement testCopyStreamToFile().
     // TOOD(huangs): Implement testReadStream().
     // TOOD(huangs): Implement testGetUriForFile().
-    // TOOD(huangs): Implement testGetExtension().
+
+    @Test
+    public void testGetExtension() {
+        assertEquals("txt", FileUtils.getExtension("foo.txt"));
+        assertEquals("txt", FileUtils.getExtension("fOo.TxT"));
+        assertEquals("", FileUtils.getExtension(""));
+        assertEquals("", FileUtils.getExtension("No_extension"));
+        assertEquals("foo_config", FileUtils.getExtension(".foo_conFIG"));
+        assertEquals("6", FileUtils.getExtension("a.1.2.3.4.5.6"));
+        assertEquals("a1z2_a8z9", FileUtils.getExtension("a....a1z2_A8Z9"));
+        assertEquals("", FileUtils.getExtension("dotAtEnd."));
+        assertEquals("ext", FileUtils.getExtension("/Full/PATH/To/File.Ext"));
+        assertEquals("", FileUtils.getExtension("/Full.PATH/To.File/Extra"));
+        assertEquals("", FileUtils.getExtension("../../file"));
+        assertEquals("", FileUtils.getExtension("./etc/passwd"));
+        assertEquals("", FileUtils.getExtension("////////"));
+        assertEquals("", FileUtils.getExtension("........"));
+        assertEquals("", FileUtils.getExtension("././././"));
+        assertEquals("", FileUtils.getExtension("/./././."));
+    }
+
     // TOOD(huangs): Implement testQueryBitmapFromContentProvider().
 }