DEFAULT_FILE_NAME_MAPPING constant is never tested

## Description

The `DEFAULT_FILE_NAME_MAPPING` constant is defined at `MappingUtils.java:72` but is never referenced in any test. Only `DEFAULT_FILE_NAME_MAPPING_CLASSIFIER` is tested (in `MappingUtilsTest.java`).

## Risk

Regressions in the default file name mapping expression would go completely undetected by the test suite.

## Suggestion

Add a test case that exercises `DEFAULT_FILE_NAME_MAPPING` similar to how `mappingWithClassifier` tests `DEFAULT_FILE_NAME_MAPPING_CLASSIFIER`.

Start at DEFAULT_FILE_NAME_MAPPING in MappingUtils.java:72 and review the mappingWithClassifier test in MappingUtilsTest.java. Add a comparable test that exercises the default file name mapping expression, then run the MappingUtilsTest suite. Done means the constant is directly covered by a passing test.

Written by the indexing model from the issue text.