luci-base: http.uc: fix invoking Lua file upload callbacks

In case a callback is installed from Lua code, the received callback
argument is not a callable function but a resource object with a call
method. Attempts to call it will leading to a `Type error: left-hand
side is not a function` error.

To solve this issue, invoke the internally set filehandler instead
which wraps the resource if needed.

Fixes: #6067
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/ucode/http.uc

@@ -432,7 +432,7 @@ const Class = {
 				let data = value.fd.read(1024);
 				let eof = (data == null || data == '');
 
-				callback(value, data, eof);
+				this.filehandler(value, data, eof);
 
 				if (eof) {
 					value.fd.close();