Avoid duplicate per-blob file ACKs

(cherry picked from commit 1c90cb120e)

static/client.html

@@ -1521,9 +1521,6 @@
             // ── File download handler (server-initiated file transfer) ──
             client.onfile = function(stream, mimetype, filename) {
                 var reader = new Guacamole.BlobReader(stream, mimetype);
-                reader.onprogress = function(length) {
-                    stream.sendAck('Received', Guacamole.Status.Code.SUCCESS);
-                };
                 reader.onend = function() {
                     var blob = reader.getBlob();
                     var a = document.createElement('a');

tests/client-file-stream-ack.mjs

@@ -36,6 +36,6 @@ assert.deepEqual(acknowledgements, [
     { message: 'OK', code: 0 }
 ], 'the stream should be accepted once the reader is ready');
 
-reader.onprogress();
-assert.deepEqual(acknowledgements.at(-1), { message: 'Received', code: 0 });
+assert.equal(reader.onprogress, undefined,
+    'BlobReader handles per-blob ACKs without an extra callback');
 console.log('client-file-stream-ack: ok');