Behavior of .slice() is incorrect when using blobs backed by a file on the disk

When using something like the `fileFrom()`, the behavior is incorrect. `.slice()` breaks if used more than once.

`blob.slice(10, 40).slice(10, 20)` should be equivalent to `blob.slice(20, 30)`, and it is if you use a blob that's backed by an arraybuffer, however when using a blob backed by a file on the disk, it becomes equivalent to `blob.slice(10, 20)` instead.