DEx_extension_only_for_class_files

Exceptiion while executing build 
FAILURE: Build failed with an exception.
* What went wrong:
Execution failed for task ':app:transformClassesWithDexBuilderForDebug'.
> com.android.build.api.transform.TransformException: java.lang.IllegalStateException: Dex archives: setting .DEX extension only for .CLASS files

Sloution :
You need to look at the build.gradle. There may be something wrong with your configuration.
You may have misspelt something or may not have configured the plugins correctly.

My issues were with the annotationProcessor  not being configured properly.