Restore the ability to save request bodies for upload requests

[englehardt]

In #322 we had to disable our file upload test which records the request body for a couple file uploads. The Firefox WebExtension API does not currently expose the content of uploaded files with the webRequest/requestBody object.

See: https://searchfox.org/mozilla-central/rev/b3b401254229f0a26f7ee625ef5f09c6c31e3949/toolkit/components/extensions/webrequest/WebRequestUpload.jsm#338-343.

See also: https://bugzilla.mozilla.org/show_bug.cgi?id=1376155#c0