Not loading a child property into memory

I need to get the list of files Ids and Names of a Post without loading their Content into Memory.

What is the correct way to get a list of Posts' Files Ids and Names without loading their Content into Memory?

Once you said you want to get a Post and then said you want to get a list of Post.

So to get a Post with its files (only Id and Name) you can write your query as follows:

var post = await _context.Posts.Where(yourCondition).Select(p => new
                {
                   p.Id,
                   p.Type
                   Files = p.Files.Select(f => new {f.Id,f.Name}).ToList()
                }).FirstOrDefaultAsync();

And to get list of Posts with its files (only Id and Name) you can write your query as follows:

var posts = await _context.Posts.Where(yourCondition).Select(p => new
                {
                   p.Id,
                   p.Type
                   Files = p.Files.Select(f => new {f.Id,f.Name}).ToList()
                }).ToListAsync();

Note: If you need strongly typed then can write as follows:

Post post = await _context.Posts.Where(yourCondition).Select(p => new Post
                    {
                       Id = p.Id,
                       Type = p.Type
                       Files = p.Files.Select(f => new File {f.Id,f.Name}).ToList()
                    }).FirstOrDefaultAsync();