Document Content-Type header values based on the type of HttpContent used

[simonrozsival]

Describe the issue or suggestion

This is a follow-up to a runtime issue dotnet/runtime#102923

The "Make HTTP requests with the HttpClient class" article hints that different HttpContent subclasses can be used for different mime types. I would prefer if the list would be a table and it would clearly state what the default value of the Content-Type header will be if the particular type is used.

Also, at least for the ByteArrayContent, it should mention that the content type needs to be set manually with an example such as:

var content = new ByteArrayContent(serialized);
content.Headers.ContentType = new MediaTypeHeaderValue("application/octet-stream");

Additionally, the article could mention that this behavior could vary based on the platform. For example, the Android default HTTP handler won't allow sending requests with a body but without a Content-Type header and if none is provided, it will default to application/x-www-form-urlencoded. I'm not sure if this article is an appropriate places for this mobile-specific information and if there isn't a better document that should cover it (maybe https://learn.microsoft.com/en-us/dotnet/maui/data-cloud/rest?view=net-maui-8.0 ?).