Don't guess mimetype based on extension for directories.

Bug: 31796976
Change-Id: I31351eea4a47ff0d3b2b81de69c1998bcb0a8274
diff --git a/src/com/android/providers/media/MediaProvider.java b/src/com/android/providers/media/MediaProvider.java
index 8929bce..80ea398 100644
--- a/src/com/android/providers/media/MediaProvider.java
+++ b/src/com/android/providers/media/MediaProvider.java
@@ -3221,7 +3221,7 @@
             }
         }
 
-        if (mimeType == null && path != null) {
+        if (mimeType == null && path != null && format != MtpConstants.FORMAT_ASSOCIATION) {
             mimeType = MediaFile.getMimeTypeForFile(path);
         }
         if (mimeType != null) {