How to set IdentityUser as a ForeignKey?

Write your UploadFile model as follows:

public class UploadFile
{

    public int Id { get; set; }
    public string UserId { get; set; } 
    public byte[] md5 { get; set; }
    public string Uri { get; set; }
    public string ThumbnialUri { get; set; }

    public ApplicationUser ApplicationUser { get; set; } //this user is extend from `IdentityUser`
}

Then in the DbConext configuration.

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
     modelBuilder.Entity<UploadFile>()
            .HasOne(uf => uf.ApplicationUser)
            .WithMany()
            .HasForeignKey(uf => uf.UserId);
}

Now run a brand new migration!

And modify your FileUpload method code as follows:

UploadFile uploadedFile = new UploadFile()
                    {
                        md5 = md5Value,
                        Uri = fileBlob.Uri.ToString(),
                        UserId = currentUser.Id, // Here set the current logged in UserId
                        ThumbnialUri = thumbnailBlob.Uri.ToString()
                    };

                    _dbContext.uploadFiles.Add(uploadedFile);

                    _dbContext.SaveChanges();

Note: If you need how to get current logged in UserId then Here it is