[kemonoparty] Duplicate file fix

[nixxquality]

Fixes #1667

Judging by the discussion in that thread, the first file is always duplicated in the attachments list, except for some posts which only have the image and no attachments.

This change makes it so if attachments are present it only downloads those.

It might require testing from various posts though. It worked in the one I tried for what it's worth, but I'm not too familiar with the service.

[mikf]

This would "break" at least Fanbox (Example) and Subscribestar (Example) posts, since the first file for those is not a duplicate of the first attachment.

Your solution could also be simplified quite a bit:

            if file and not post["attachments"]:

[nixxquality]

I wasn't familiar with that syntax of checking a python list's emptiness before. This should be good.

[mikf]

I've also been looking for patreon posts where this wouldn't work and haven't found any for now.
The closest was this one (nsfw) where main file and second attachment are identical, instead of the first.

[nixxquality]

I think the shorter example for the fix you posted earlier was wrong and I applied it without thinking.

I've flipped the post["attachments"] check now; it should be skipping the main file when there are attachments.

I would have liked to add some simple test cases but I'm not familiar with Python and I'm not sure where they would go, or if you think they are necessary.