Fix content uploads for modern browsers
Modern browsers now expose a `stream` function on the Blob and File interfaces.
This conflicts with an older style of passing data to the `uploadContent` SDK
method, which supported supplying the data to upload in the `stream` property of
an object.
Since this old style is still in active use in the Matrix JS ecosystem, we
preserve the backwards compatibility for now by checking whether `stream` is a
function.
This fix has been tested in Firefox Nightly (69), Firefox Release (67), Chrome
Canary (77), and Chrome Stable (75).
Fixes https://github.com/vector-im/riot-web/issues/9913
Fixes https://github.com/matrix-org/matrix-js-sdk/issues/949
