Added implementation for url_file_attachment option.

Using url_file_attachment user can specify if he like to change objects
like 'file_share' marked as external (in Slack servers point of view)
to attachments, so the only value would be remembered URL for the
"uploaded" document as an attachment. Or, treat it as is, and let the
slack-backup to produce file which contain the URLs and corresponding
local file names for such files.

slack_backup/client.py

@@ -6,6 +6,7 @@ import json
 import logging
 import os
 import pprint
+import uuid
 
 import slackclient
 import sqlalchemy.orm.exc
@@ -46,6 +47,13 @@ class Client(object):
         if 'format' in args:
             self.reporter = reporters.get_reporter(args, self.q)
 
+        self._url_file_to_attachment = args.url_file_to_attachment
+
+        self._dlpath = utils.get_temp_name(dir=os.path.curdir,
+                                           prefix='manual_download_',
+                                           unlink=True)
+        self._dldata = []
+
     def update(self):
         """
         Perform an update, store data to db
@@ -54,6 +62,7 @@ class Client(object):
         self.update_users()
         self.update_channels()
         self.update_history()
+        self._finalize()
 
     def update_channels(self):
         """Fetch and update channel list with current state in db"""
@@ -210,19 +219,31 @@ class Client(object):
                 message.reactions.append(o.Reaction(reaction_data))
 
         if data.get('subtype') == 'file_share':
-            self._file_data(message, data['file'], data['file']['is_external'])
+            if (self._url_file_to_attachment and
+                    data['file'].get('is_external')):
+                fdata = data['file']
+                # change message type from file_share to default
+                message.type = ''
+                message.text = (message.text.split('shared a file:')[0] +
+                                'shared a file: ')
+                logging.debug("Found external file `%s'. Saving as "
+                              "attachment.", fdata['url_private'])
+                self._att_data(message, [{'title': fdata['name'],
+                                          'text': fdata['url_private'],
+                                          'fallback': ''}])
+            else:
+                self._file_data(message, data['file'])
         elif data.get('subtype') == 'pinned_item':
             if data.get('attachments'):
                 self._att_data(message, data['attachments'])
             elif data.get('item'):
-                self._file_data(message, data['item'],
-                                data['item']['is_external'])
+                self._file_data(message, data['item'])
         elif data.get('attachments'):
             self._att_data(message, data['attachments'])
 
         self.session.add(message)
 
-    def _file_data(self, message, data, is_external=True):
+    def _file_data(self, message, data):
         """
         Process file data. Could be either represented as 'file' object or
         'item' object in case of pinned items
@@ -231,8 +252,15 @@ class Client(object):
         if data.get('is_starred'):
             message.is_starred = True
 
-        if is_external:
-            logging.debug("Found external file `%s'", data['url_private'])
+        if data.get('is_external'):
+            # Create a link and corresponding file name for manual download
+            fname = str(uuid.uuid4())
+            message.file.filepath = self.downloader.get_filepath(fname, 'file')
+            logging.info("Please, manually download an external file from "
+                         "URL `%s' to `%s'", data['url_private'],
+                         message.file.filepath)
+            self._dldata.append('%s --> %s\n' % (data['url_private'],
+                                                 message.file.filepath))
             message.file.url = data['url_private']
         else:
             logging.debug("Found internal file `%s'",
@@ -354,3 +382,14 @@ class Client(object):
                 return result['messages'], None
 
         return [], None
+
+    def _finalize(self):
+        """Create misc files if necessary - like manual donwload"""
+        if not self._dldata:
+            return
+
+        with open(self._dlpath, "a") as fobj:
+            fobj.write(''.join(self._dldata))
+        logging.warning("Manual action required! Download all the files "
+                        "listed in `%s' and each of them save as file listed "
+                        "right after `-->' sign", self._dlpath)