Index: applications/editors/josm/plugins/mapillary/src/org/openstreetmap/josm/plugins/mapillary/utils/ImageUtil.java
===================================================================
--- applications/editors/josm/plugins/mapillary/src/org/openstreetmap/josm/plugins/mapillary/utils/ImageUtil.java	(revision 32631)
+++ applications/editors/josm/plugins/mapillary/src/org/openstreetmap/josm/plugins/mapillary/utils/ImageUtil.java	(revision 32642)
@@ -146,9 +146,12 @@
       try (FileInputStream fis = new FileInputStream(f)) {
         int numBytes = fis.read(magic);
-        return Arrays.equals(JFIF_MAGIC, Arrays.copyOf(magic, Math.min(numBytes, JFIF_MAGIC.length)))
-            || Arrays.equals(PNG_MAGIC, Arrays.copyOf(magic, Math.min(numBytes, PNG_MAGIC.length)));
+        return numBytes >= 0 && (
+          Arrays.equals(JFIF_MAGIC, Arrays.copyOf(magic, Math.min(numBytes, JFIF_MAGIC.length)))
+          || Arrays.equals(PNG_MAGIC, Arrays.copyOf(magic, Math.min(numBytes, PNG_MAGIC.length)))
+        );
       } catch (FileNotFoundException e) {
         return false;
       } catch (IOException e) {
+        Main.warn("IO-exception while reading file "+f.getAbsolutePath());
         return false;
       }