We could merge the two top-most matches and remove unnecessary clone like this:

let (range_start, range_end): (u64, u64) = match &range_header {
    Some(web::Header(Range::Bytes(ranges))) => {
      if ranges.len() > 1 {
        return Err(ErrorBadRequest("Multiple ranges not supported"));
      }
      <...rest of the code...>
   }
   Some(web::Header(Range::Unregistered(..))) => {
      return Err(ErrorBadRequest("Use ranges registered at IANA"));
    }
    None => (0, file_size - 1),
};

which I personally find a little easier to read

In D8367#246679, @michal wrote:

We could merge the two top-most matches and remove unnecessary clone like this:
...
which I personally find a little easier to read

Yes, good call. That is indeed an improvement worth doing

Nit: I don't we need the two // Parse range header comments, now that we have a function with the same name

A small thing, and shouldn't matter performance-wise, but you should be able to take the range_header variable as a reference in the function, instead of taking it as an owned value and cloning it with to_owned().

In general, it's better to take references as arguments in functions (except for the Copy types) if it is possible. As a caller, you can always go from owned value to a reference, but not always the other way around or it could cost an additional clone.

Nothing stands out to me that already hasn't been addressed.