为什么我需要 Content/Type 变量？

Question

In my MVC Core application, I have in my controller a way for the user to download an image that I have saved which is called from a button click in my view. I don't understand why I need the second variable "contentType" string in my return File(). I can still download the image as a.jpg if I put in "application/pdf" instead of "application/jpg", so what is even the point of having it other than the fact that it is required. What is the point of this variable?

public FileStreamResult Download()
    {
        string filename = "Capture.JPG";
        string filepath = hostingEnvironment.WebRootPath + "\\Image";
        string fullName = Path.Combine(filepath, filename);
        return File(new FileStream(fullName, FileMode.Open), "application/pdf", filename);
    }

Answer 1

Flydog's comment is quite pertinent; perhaps the most useful or obvious difference you'll find in declaring a content type is that the browser may use it to invoke a different behavior depending on the setting. If you send an mp4 video as "video/mp4" the browser may well play it within its viewport. If you send the same file as "application/octet-stream" it may offer a save as. If you claim one content type and it's actually another it's likely the browser will ignore you (note: claiming a pdf is pdf or octet stream are both reasonable/true. Claiming it's a jog is not). Content-Disposition also plays a part in the server aiming to give the browser some direction on what to do with the file..

..but primarily I'd say a good reason to tell the truth about the content type (ie don't claim it's a jpg when it's a pdf) is the interop mantra "when it sones to standards adherence, be strict in what you send and liberal in what you accept" - you're sending, so follow protocol:)