The example is unnatural and probably does not correspond to naming conventions (so probably not an issue); but just in case:

please take JDK-8133847 test example and define
private static final String IMAGE_NAME_1X = "someimage";
private static final String IMAGE_NAME_2X = "someimage@2x";

Output (Mac OS X 10.10.3, JDK9 b77) is as expected:
  file: /Users/xxx/work/tests/Test/someimage
  image class: sun.awt.image.MultiResolutionToolkitImage

then set
private static final String IMAGE_NAME_1X = "some.image";
private static final String IMAGE_NAME_2X = "some.image@2x";

Output is invalid:
  file: /Users/xxx/work/tests/Test/some.image
  image class: sun.awt.image.ToolkitImage
   Exception in thread "main" java.lang.ClassCastException: sun.awt.image.ToolkitImage cannot be cast to sun.awt.image.MultiResolutionImage
        at Test.main(Test.java:27)

(in case of non-empty extension the MultiResolutionImage could be read).