[dropbox] Recognize 'https://www.dropbox.com/sh/*' urls (fixes #3795)

And extract the title from the url last path component.

youtube_dl/extractor/dropbox.py

@@ -5,24 +5,29 @@ import os.path
 import re
 
 from .common import InfoExtractor
-from ..utils import compat_urllib_parse_unquote
+from ..utils import compat_urllib_parse_unquote, url_basename
 
 
 class DropboxIE(InfoExtractor):
-    _VALID_URL = r'https?://(?:www\.)?dropbox[.]com/s/(?P<id>[a-zA-Z0-9]{15})/(?P<title>[^?#]*)'
-    _TEST = {
+    _VALID_URL = r'https?://(?:www\.)?dropbox[.]com/sh?/(?P<id>[a-zA-Z0-9]{15})/.*'
+    _TESTS = [{
         'url': 'https://www.dropbox.com/s/nelirfsxnmcfbfh/youtube-dl%20test%20video%20%27%C3%A4%22BaW_jenozKc.mp4?dl=0',
         'info_dict': {
             'id': 'nelirfsxnmcfbfh',
             'ext': 'mp4',
             'title': 'youtube-dl test video \'ä"BaW_jenozKc'
         }
-    }
+    },
+    {
+        'url': 'https://www.dropbox.com/sh/662glsejgzoj9sr/AAByil3FGH9KFNZ13e08eSa1a/Pregame%20Ceremony%20Program%20PA%2020140518.m4v',
+        'only_matching': True,
+    },
+    ]
 
     def _real_extract(self, url):
         mobj = re.match(self._VALID_URL, url)
         video_id = mobj.group('id')
-        fn = compat_urllib_parse_unquote(mobj.group('title'))
+        fn = compat_urllib_parse_unquote(url_basename(url))
         title = os.path.splitext(fn)[0]
         video_url = (
             re.sub(r'[?&]dl=0', '', url) +