Previously, `getFile()` was called indiscriminately during uploads, moves,
and download link generation. This could lead to users with download limit
causing subsequent operations like uploads and moves to fail.
This PR optimizes the use of getFile(), by only calling it
when it's strictly necessary.
Before this change, the Pikpak backend would always download
the first media item whenever possible, regardless of whether
or not it was the original contents.
Now we check the validity of a media link using the `fid`
parameter in the link URL.
Fixes#6992
