Harmonize file filter logic for File inputs

[imagejan]

We currently have inconsistent logic when setting up a file filter for File and File[] inputs.

The extensions style is only supported for File[] inputs, not for File, see:

scijava-ui-swing/src/main/java/org/scijava/ui/swing/widget/SwingFileWidget.java

Line 137 in 08fb795

file = uiService.chooseFile(file, style);

Actually, the file filter seems to work for drag-and-drop (in FileHandler):

scijava-ui-swing/src/main/java/org/scijava/ui/swing/widget/SwingFileWidget.java

Lines 357 to 358 in 08fb795

	final List<File> files = SwingFileWidget.filterFiles(allFiles,
	filter);

... but not for the Browse button.

Related scijava-common issue: scijava/scijava-common#197