Index: /trunk/src/org/openstreetmap/josm/data/imagery/WMTSTileSource.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/imagery/WMTSTileSource.java	(revision 11823)
+++ /trunk/src/org/openstreetmap/josm/data/imagery/WMTSTileSource.java	(revision 11824)
@@ -409,4 +409,5 @@
         List<String> supportedMimeTypes = new ArrayList<>(Arrays.asList(ImageIO.getReaderMIMETypes()));
         supportedMimeTypes.add("image/jpgpng");         // used by ESRI
+        supportedMimeTypes.add("image/png8");           // used by geoserver
         Collection<String> unsupportedFormats = new ArrayList<>();