fix crash on unknown media type

app/src/main/java/com/keylesspalace/tusky/fragment/ViewMediaFragment.kt

@@ -48,7 +48,7 @@ abstract class ViewMediaFragment : BaseFragment() {
                 Attachment.Type.IMAGE -> ViewImageFragment()
                 Attachment.Type.VIDEO,
                 Attachment.Type.GIFV -> ViewVideoFragment()
-                else -> throw Exception("Unknown media type: $attachment")
+                else -> ViewImageFragment()   // it probably won't show anything, but its better than crashing
             }
             fragment.arguments = arguments
             return fragment