Fix generated file extension for Android

android/src/main/java/fr/greweb/reactnativeviewshot/RNViewShotModule.java

             cacheDir = externalCacheDir.getFreeSpace() > internalCacheDir.getFreeSpace() ?
                     externalCacheDir : internalCacheDir;
         }
-        return File.createTempFile(TEMP_FILE_PREFIX, ext, cacheDir);
+        String suffix = "." + ext;
+        return File.createTempFile(TEMP_FILE_PREFIX, suffix, cacheDir);
     }
 
 }