[kemonoparty] Duplicate file fix

Fixes mikf#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.

gallery_dl/extractor/kemonoparty.py

@@ -51,15 +51,20 @@ def items(self):
             files = []
             append = files.append
             file = post["file"]
+            got_attachment = False
 
             if file:
                 file["type"] = "file"
                 append(file)
             for attachment in post["attachments"]:
                 attachment["type"] = "attachment"
+                got_attachment = True
                 append(attachment)
             for path in find_inline(post["content"] or ""):
                 append({"path": path, "name": path, "type": "inline"})
+
+            if got_attachment:
+                files = files[1:] # ignore the first file if there are attachments
 
             post["date"] = text.parse_datetime(
                 post["published"], "%a, %d %b %Y %H:%M:%S %Z")