set background dark/light theme aware for upload bottom sheet

Resolves #994

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

app/src/main/res/layout/dialog_attachment.xml

@@ -21,6 +21,7 @@
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
+    android:background="@color/bg_bottom_sheet"
     android:orientation="vertical"
     android:paddingBottom="8dp">