[postprocessor:classify] improve path generation (fixes #138)

It still doesn't work for converted ugoira animations thanks to how
those files are handled, but everything else, including files with
unknown or changing file extension, now works as it should.

gallery_dl/postprocessor/classify.py

@@ -34,16 +34,24 @@ class ClassifyPP(PostProcessor):
 
     def prepare(self, pathfmt):
         ext = pathfmt.extension
-
         if ext in self.mapping:
-            self._dir = pathfmt.realdirectory + os.sep + self.mapping[ext]
-            pathfmt.realpath = self._dir + os.sep + pathfmt.filename
-        else:
-            self._dir = None
+            # set initial paths to enable download skips
+            self._build_paths(pathfmt, self.mapping[ext])
 
     def run(self, pathfmt):
-        if self._dir:
-            os.makedirs(self._dir, exist_ok=True)
+        ext = pathfmt.extension
+        if ext in self.mapping:
+            # rebuild paths in case the filename extension changed
+            path = self._build_paths(pathfmt, self.mapping[ext])
+            os.makedirs(path, exist_ok=True)
+
+    @staticmethod
+    def _build_paths(pathfmt, extra):
+        path = pathfmt.realdirectory + os.sep + extra
+        pathfmt.realpath = path + os.sep + pathfmt.filename
+        pathfmt.path = (pathfmt.directory + os.sep +
+                        extra + os.sep + pathfmt.filename)
+        return path
 
 
 __postprocessor__ = ClassifyPP