Changed behavoiur for duplicates

Till now, if we download certain files (like those attached to the
conversation) and we already have the file with the same name, number in
format '%03d' was added just before extension. That way there could be
possibility, that the very same file will be downloaded and stored
multiple times, like:

file.png
file.001.png
file.002.png
...

This commit prevents that by adding comparison between files we already
have and file which is downloaded from slack. Adding another file with
additional number will only have place when stored file and downloaded
have different content.

slack_backup/download.py

@@ -5,6 +5,7 @@ to local ones, so that sophisticated writers can make a use of it
 import functools
 import logging
 import os
+import shutil
 
 import requests
 
@@ -67,10 +68,26 @@ class Download(object):
         if not self._hier_created:
             self._create_assets_dir()
 
-        fname = self.prepare_filepath(url, filetype)
+        filepath = self.get_filepath(url, filetype)
+        temp_file = utils.get_temp_name()
 
-        self._download(url, fname)
-        return fname
+        self._download(url, temp_file)
+
+        if filepath and os.path.exists(filepath) and filetype != 'avatar':
+            if not utils.same_files(filepath, temp_file):
+                logging.warning("File `%s' already exist, renamed to `%s'",
+                                filepath,
+                                self.calculate_new_filename(filepath,
+                                                            filetype))
+                filepath = self.calculate_new_filename(filepath, filetype)
+                shutil.move(temp_file, filepath)
+            else:
+                logging.debug("File `%s' already exist, skipping", filepath)
+                os.unlink(filepath)
+        else:
+            shutil.move(temp_file, filepath)
+
+        return filepath
 
     def _create_assets_dir(self):
         for path in (self._files, self._images):
@@ -78,8 +95,8 @@ class Download(object):
 
         self._hier_created = True
 
-    def prepare_filepath(self, url, filetype):
-        """Prepare directory where to download file into"""
+    def get_filepath(self, url, filetype):
+        """Get full path and filename for the file"""
 
         typemap = {'avatar': self._images,
                    'file': self._files}
@@ -91,7 +108,8 @@ class Download(object):
 
         splitted = url.split('/')
 
-        if len(splitted) == 7 and 'slack.com' in splitted[2]:
+        if len(splitted) == 7 and ('slack.com' in splitted[2] or
+                                   'slack-edge.com' in splitted[2]):
             part = url.split('/')[-3]
             fname = url.split('/')[-1]
         else:
@@ -105,7 +123,9 @@ class Download(object):
             utils.makedirs(os.path.join(path, part))
             path = os.path.join(path, part)
 
-        path = os.path.join(path, fname)
+        return os.path.join(path, fname)
+
+    def calculate_new_filename(self, path, filetype):
         count = 1
 
         while filetype != 'avatar' and os.path.exists(path):