Conditional Include() in Entity Framework

Remove the virtual keyword from your Attachments navigation property to prevent lazy loading:

public ICollection<Attachment> Attachments { get; set; }

First method: Issue two separate queries: one for the Posts, one for the Attachments, and let relationship fix-up do the rest:

List<Post> postsWithAuthoredAttachments = _context.Posts
    .Include(p => p.Comments) 
    .Include(p => p.PostAuthor)
    .Where(p => p.PostAuthor.Id == postAuthorId)
    .ToList();

List<Attachment> filteredAttachments = _context.Attachments
    .Where(a => a.Post.PostAuthor.Id == postAuthorId)
    .Where(a => a.Owner is Author)
    .ToList()

Relationship fixup means that you can access these filtered Attachments via a Post's navigation property

Second method: one query to the database followed by an in-memory query:

var query = _context.Posts
    .Include(p => p.Comments) 
    .Include(p => p.PostAuthor)
    .Where(p => p.PostAuthor.Id == postAuthorId)
    .Select(p => new 
        {
            Post = p,
            AuthoredAttachments = p.Attachments
                Where(a => a.Owner is Author)
        }
    );

I would just use the anonymous type here

var postsWithAuthoredAttachments = query.ToList()

or I would create a ViewModel class to avoid the anonymous type:

List<MyDisplayTemplate> postsWithAuthoredAttachments = 
     //query as above but use new PostWithAuthoredAttachments in the Select

Or, if you really want to unwrap the Posts:

List<Post> postsWithAuthoredAttachments = query.//you could "inline" this variable
    .AsEnumerable() //force the database query to run as is - pulling data into memory
    .Select(p => p) //unwrap the Posts from the in-memory results
    .ToList()