Fixes Clippy lints for:
* src/thumbnailer.rs
* src/operation/mod.rs
* src/operation/recursive.rs
One of the Clippy lints also involved fixing a TODO to replace a
deprecated function that malfunctioned under Windows.
Closes: #698
I ended up fixing a few Clippy lints while tracking down this problem.
It turns out that the issue was way simpler than I assumed: ".tar.xz"
needed to be added to a slice of extensions to strip.
