rails/marcel: Find the mime type of files, examining file, filename and declared type

Marcel chooses the most appropriate content type for a file by inspecting its contents, the declared MIME type (perhaps passed as a Content-Type header), and the file extension. Marcel checks, in order: The "magic bytes" sniffed from the file contents. The declared type, typically provided in a Content-Type header on an uploaded file, unless it's the application/octet-stream default. The filename extension. Safe fallback to the indeterminate application/octet-stream default.