How to save image data in session?

Question

Sessions

I have a field that saves images made with dropzonejs .
Files are sent by Ajax to a server and then stored in temporary storage. How can I save data about a file into a session?
When I write:

Session::put('topicCover', $name);

server returns an error. It simply does not allow writing anything to the session.
The error is this in the console:

Failed to load resource: the server responded with a status of 500 (Internal Server Error)