Difference between multipart form upload and NSURLSession.uploadTaskWithRequest

The uploadTaskWithRequest simply sends the NSData, file, or stream as the body of the request. It doesn't do anything beyond that. It simply has the benefit that it can be used with background sessions.

So, if you have web service that is expecting multipart/form-data request, you have to build that request yourself (unless you use something like AFNetworking or Alamofire to do this for you). Once you've built that request, you can either use dataTaskWithRequest (having set the HTTPBody of the NSMutableURLRequest) or uploadTaskWithRequest (in which case you don't set HTTPBody, but rather provide it as a parameter to uploadTaskWithRequest).

By the way, a tool like Charles is very useful in these cases, letting you observe what's going on behind the scenes.

File Upload with multipart/form-data

The first approach using a multipart/form-data Content-type was originally defined in RFC 1867, then moved to the World Wide Web Consortium, which included it in the specification for HTML 4.0, where forms are expressed in HTML and where form values are sent via HTTP and electronic mail. When the form has been filled out by a user the form was sent to the server. This technique is widely supported and used by browsers and web servers.

However multipart/form-data can also be used to define form data which are presented in other representations than HTML. That is, you don't necessarily need a web browser or web server. The current specification which can be used by a wide variety of applications and transported by a wide variety of protocols is RFC 7578 (form IETF).

It must be mentioned, though, that the multipart/form-data content type was not always/is not without issues. It is quite complex by itself. Additionally, it uses/refers to a lot of other RFCs and - as a result of clearing things up - it and those which it depends on have been changed, obsoleted and updated quite frequently. Due to its complexity, serialisers and parsers are getting quite complicated, too and there's a lot of room for bugs and other issues.

NSURLSession uploadTaskWithRequest

How NSURLSession composes a request is not precisely documented. It certainly does not use a multipart/form-data content type, though.

For upload tasks, NSURLSession uses a POST request with a NSURLRequest as parameter which you can setup yourself. That is, you optionally can set the content type (for example text/plain; charset=utf-8), and other headers. NSURLSession can also derive an appropriate content type itself from the given content (file, stream or NSData). That is, we may say, it becomes a "simple" POST request. Due to less complexity, the request is less troublesome.

So, in order for your server to support an uploadTaskWithRequest where a file should be uploaded, it should simply support a POST request with some "simple" content type. That is, as opposed to a "file upload" with a multipart/form-data content type which contains the file name in a disposition header, the server would need to return the URL of the location where the resource (the file) has been written to.