Thumbnails are back to matching the source extension for now, for compat with V1

Before changing it back, need to list the rules for extension changes. For example:

- should GIF thumbnail should be JPG, to avoid animations on the album page?
- what about transparent GIFs, will they look weird in JPG?
- maybe GIFs should stay as GIFs, but kept to a single frame only for thumbnails
- same thing for pngs, which might be better kept as PNG for transparency
- all other non-browser-friendy formats should become JPG

These rules will be a lot easier to implement when the new input data structure is in place

src/index.js

@@ -81,7 +81,7 @@ exports.build = function(opts) {
     buildStep({
       message: 'Photos: thumbnails',
       ext:     'jpg|jpeg|png|gif',
-      dest:    '/thumbs/$path/$name.jpg',
+      dest:    '/thumbs/$path/$name.$ext',
       func:    thumbs.photoSquare
     }),
 

src/model/file.js

@@ -30,7 +30,7 @@ function videoUrls(filepath) {
 
 function photoUrls(filepath) {
   return {
-    thumb:     'media/thumbs/' + ext(filepath, 'jpg'),
+    thumb:     'media/thumbs/' + filepath,
     large:     'media/large/' + filepath,
     original:  'media/original/' + filepath
   };