Make file filter show the extension, instead of cryptic stuff

* Override FileFilter toString() method in SwingFileWidget so that it shows the extension(s) that are filtered by the file choose dialog instead of the standard toString() of the
* Related to
  * bigdataviewer/bigdataviewer-playground#110
  * https://forum.image.sc/t/scijava-command-file-extensions/29869/2

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

@@ -40,6 +40,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import java.util.StringJoiner;
 import java.util.stream.Collectors;
 
 import javax.swing.Box;
@@ -235,6 +236,18 @@ public boolean accept(final File pathname) {
 				}
 				return false;
 			}
+
+			@Override
+			public String toString()
+			{
+				List< String > extsWithPrefix = new ArrayList<>();
+				for ( String ext : exts )
+					extsWithPrefix.add( "*." + ext );
+
+				StringJoiner joiner = new StringJoiner( ";" );
+				extsWithPrefix.forEach( joiner::add );
+				return joiner.toString();
+			}
 		};
 	}