添付メディア背景も角丸に

app/src/main/res/drawable/bg_thumbnail.xml

@@ -3,5 +3,10 @@
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:color="?attr/colorRippleEffect"
     >
-    <item android:drawable="?attr/colorThumbnailBackground"/>
+    <item>
+        <shape android:shape="rectangle">
+            <solid android:color="?attr/colorThumbnailBackground" />
+            <corners android:radius="6dp" />
+        </shape>
+    </item>
 </ripple>