How to access file inside C# server method without uploading it to a vault.

Pre-validating a file is possible when you separate the client file upload from the actual upload to the Innovator file vault. When the file comes from a client, you should not need a C# Method.

Maybe the code samples in this discussion already help to solve your task:

community.aras.com/.../how-to-add-restrictions-to-aras-vault-selectfile-file-picker

You can do various checks before the actual upload, e.g. check file size, read file content, etc.