This attempts to solve several deficiencies in the script:
1. If the first attachment of a bug is already downloaded, the bug is
   not checked for newly added attachments (or attachments with newly
   fixed mimetype).
2. If neither of the eligible attachment(s) of a bug is the first
   attachment, the bug will be processed (and the attachment downloaded)
   time and again (because the shortcut is only applied for the first
   attachment, see 1).

But it also ensures that if the script is killed, the download is
restarted on the next run.

Change-Id: I7f3d1922825bb314f96ec3b1ee2a0ac47604b018