Why not MIT License? IANAL but I hear the WTFPL doesn't protect the developer.

[Poikilos]

Sources:

• https://law.stackexchange.com/questions/29718/is-the-wtfpl-globally-valid
• "As a director of an open source program office, I make sure that we (Oath, used to be Yahoo) do not publish code using WTFPL since I want people to use our open source code. It’s important to me since our code is important to them." -Gil Yehuda's answer at https://www.quora.com/The-WTFPL-license-seems-like-a-great-license-to-use-as-it-gives-full-freedom-Why-are-there-some-people-that-oppose-this-license
• https://cubicspot.blogspot.com/2017/04/wtfpl-is-harmful-to-software-developers.html

Since the upstream license says you can do whatever you want, changing the license seems ok since that is a thing you might want to do, but again IANAL.

The same goes for rust-ffmpeg-sys: Though the upstream license file is missing it is specified here: https://github.com/meh/rust-ffmpeg-sys/blob/master/Cargo.toml