Include `files` param in the snippet create mutation request

This issue is related to #217932 (closed)

When we are creating a snippet and we upload a file to the description, we need to pass the existing files param to the snippet create mutation. If we don't pass it the uploaded files will be forever attached to the user instead of the personal snippet jeopardizing the availability of that file if the user is removed (more info in #217932 (closed)).

Notice that, when no files are uploaded in the creation, the files input param is not present.

/cc @dmishunov @rkuba