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>