Add files param to Snippets create mutation

At the moment, in the HAML version, when we upload a file to the snippet description and we submit the form, a files param is added to the request. We need to update the Snippet create mutation to accept that param as well.

Why? Because, when we are creating a new personal snippet and we upload a file, that attachment is assigned to the personal snippet author. Then after we submit the form and create the snippet, using that files param we reassign the attachment to the personal snippet instead of the user.

It's important that we keep that workflow, because if the user is deleted, for example, the user uploads are removed as well. If we reassign the attachment to the snippet, even if the author is deleted, the file will still remain.

We need to add a files string array argument in the snippet create mutation