Set default channels based on image mode

lib/psd/image.coffee

@@ -31,17 +31,15 @@ module.exports = class Image extends Module
     @startPos = @file.tell()
     @endPos = @startPos + @length
 
-    @channelsInfo = [
-      {id: 0}
-      {id: 1}
-      {id: 2}
-    ]
-
-    @channelsInfo.push {id: -1} if @channels() is 4
+    @setChannelsInfo()
 
   for attr in ['width', 'height', 'channels', 'depth', 'mode'] then do (attr) =>
     @::[attr] = -> @header[attr]
 
+  setChannelsInfo: ->
+    switch @mode()
+      when 3 then @setRgbChannels()
+
   calculateLength: ->
     @length = switch @depth()
       when 1 then (@width() + 7) / 8 * @height()

lib/psd/image_modes/rgb.coffee

 module.exports =
+  setRgbChannels: ->
+    @channelsInfo = [
+      {id: 0}
+      {id: 1}
+      {id: 2}
+    ]
+
+    @channelsInfo.push {id: -1} if @channels() is 4
+
   combineRgbChannel: ->
     rgbChannels = @channelsInfo
       .map (ch) -> ch.id
-      .filter (ch) -> ch >= -1
+      .filter (ch) -> ch >= -1 # Mask data is -2
 
     for i in [0...@numPixels]
       r = g = b = 0