Preserve old Content-Type behaviour for bundles

Problem: bundles may be provided by servers with a Content-Type of
application/octet-stream instead of application/vnd.olpc-sugar, which
when passed into the Journal prevents quick bundle installation.

Solution: restore Browse-157.3 behaviour by detecting bundles and
ignoring the Content-Type from the server.

Regression introduced in Browse-200, did not occur in Browse-157.3.

References:
https://bugs.sugarlabs.org/ticket/4977

Signed-off-by: James Cameron <quozl@laptop.org>

downloadmanager.py

@@ -27,6 +27,7 @@
 from gi.repository import Gdk
 from gi.repository import GdkPixbuf
 from gi.repository import GObject
+from gi.repository import Gio
 
 from sugar3.datastore import datastore
 from sugar3 import profile
@@ -213,7 +214,10 @@ def __download_finished_cb(self, download):
         self.dl_jobject.metadata['progress'] = '100'
         self.dl_jobject.file_path = self._dest_path
 
-        mime_type = download.get_response().get_mime_type()
+        mime_type = Gio.content_type_guess(self._dest_path)[0]
+        if mime_type != 'application/vnd.olpc-sugar':
+            mime_type = download.get_response().get_mime_type()
+
         self.dl_jobject.metadata['mime_type'] = mime_type
 
         if mime_type in ('image/bmp', 'image/gif', 'image/jpeg',