fix gif NPE

// FREEBIE

src/org/thoughtcrime/securesms/mms/AttachmentManager.java

@@ -88,7 +88,7 @@ public class AttachmentManager {
   }
 
   public void setImage(MasterSecret masterSecret, Uri image) throws IOException, BitmapDecodingException {
-    if (MediaUtil.getMimeType(context, image).startsWith("image/gif")) {
+    if (MediaUtil.isGif(MediaUtil.getMimeType(context, image))) {
       setMedia(new GifSlide(context, masterSecret, image), masterSecret);
     } else {
       setMedia(new ImageSlide(context, masterSecret, image), masterSecret);

src/org/thoughtcrime/securesms/util/MediaUtil.java

@@ -84,7 +84,7 @@ public class MediaUtil {
     return type;
   }
 
-  private static boolean isGif(String contentType) {
+  public static boolean isGif(String contentType) {
     return !TextUtils.isEmpty(contentType) && contentType.trim().equals("image/gif");
   }