When using File Choice Parameter with active Empty Choice setting the parameter list cannot be generated because of
java.lang.UnsupportedOperationException
at java.util.AbstractList.add(AbstractList.java:148)
at jp.ikedam.jenkins.plugins.extensible_choice_parameter.FilenameChoiceListProvider.getFileList(FilenameChoiceListProvider.java:333)

No values can be added to a list returned by Arrays.asList.