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;
@@ -235,9 +236,26 @@ public boolean accept(final File pathname) {
 				}
 				return false;
 			}
+
+			@Override
+			public String toString()
+			{
+				return formatFileFilterExtensions( exts );
+			}
 		};
 	}
 
+	static String formatFileFilterExtensions( final List< String > extensions )
+	{
+		List< String > extsWithPrefix = new ArrayList<>();
+		for ( String extension : extensions )
+			extsWithPrefix.add( "*." + extension );
+
+		StringJoiner joiner = new StringJoiner( ";" );
+		extsWithPrefix.forEach( joiner::add );
+		return joiner.toString();
+	}
+
 	/**
 	 * Checks whether the given drag and drop operation offers a list of files as
 	 * one of its flavors.

src/test/java/org/scijava/ui/swing/widget/SwingFileWidgetTest.java

@@ -0,0 +1,19 @@
+package org.scijava.ui.swing.widget;
+
+import junit.framework.TestCase;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+
+public class SwingFileWidgetTest extends TestCase
+{
+
+	public void testFormatFileFilterExtensions()
+	{
+		final List< String > multipleExtensions = Arrays.asList( "jpg", "jpeg", "png", "gif", "bmp" );
+		final List< String > singleExtension = Collections.singletonList( "xml" );
+		assertEquals( "*.jpg;*.jpeg;*.png;*.gif;*.bmp", SwingFileWidget.formatFileFilterExtensions( multipleExtensions ) );
+		assertEquals( "*.xml", SwingFileWidget.formatFileFilterExtensions( singleExtension ) );
+	}
+}