feat: revert: store attachments with the card rather than as a file

Files stored with the card don't appear as covers, nor can they be viewed within the deck app, only downloaded.

src/nextcloud/client.rs

@@ -228,7 +228,7 @@ impl<'ctx> DeckClient<'ctx> {
         APIResult::new(
             with_exponential_backoff!(&self.ctx, {
                 let form: multipart::Form = multipart::Form::new();
-                let form = form.text("type", "deck_file");
+                let form = form.text("type", "file");
                 let form = form
                     .file("file", file.as_pathbuf())
                     .await