Your solution seems fine by me. If you're looking for a nicer syntax, how about rolling it into a function like this (without the possibly gratuitous type annotations):
let await<'a> (op: IAsyncOperation<'a>) : Async<'a> =
op.AsTask() |> Async.AwaitTask
This will give you the almost exact same syntax you'd see in c#:
async {
let! file = await <| StorageFile.GetFileFromApplicationUriAsync(uri)
...
}
The compiler errors you were getting with your previous approaches are to be expected. All async workflow cares about is the F#-specific Async type. This type gives you a way to interop with the rest of .NET world through Tasks, but that's it. IAsyncOperation is from a 'different part of the world', I wouldn't expect F# core libraries to support it anytime soon.