Add support for plain control types

Closes: #1193

src/FSharp.Data.Http/Http.fs

@@ -636,6 +636,11 @@ module HttpStatusCodes =
 
 
 type MultipartItem = MultipartItem of formField: string * filename: string * content: Stream
+type MultipartFileItem = MultipartFileItem of formField: string * filename: string option * contentType: string option * content: Stream
+
+type MultipartFormDataItem =
+    | FileValue of MultipartFileItem
+    | FormValue of string * string
 
 /// The body to send in an HTTP request
 type HttpRequestBody =
@@ -649,6 +654,9 @@ type HttpRequestBody =
     /// A sequence of formParamName * fileName * fileContent groups
     | Multipart of boundary: string * parts: seq<MultipartItem>
 
+    /// A sequence of formParamName * fileName * fileContent groups
+    | MultipartFormData  of boundary: string * parts: seq<MultipartFormDataItem>
+
 /// The response body returned by an HTTP request
 type HttpResponseBody =
     | Text of string
@@ -1521,7 +1529,7 @@ module internal HttpHelpers =
     ///         c) write newline
     ///         d) write section data
     ///     3) write trailing boundary
-    let writeMultipart (boundary: string) (parts: seq<MultipartItem>) (e: Encoding) =
+    let writeMultipart (boundary: string) (parts: seq<MultipartFileItem>) (e: Encoding) =
         let newlineStream () =
             new MemoryStream(e.GetBytes "\r\n") :> Stream
 
@@ -1545,17 +1553,20 @@ module internal HttpHelpers =
 
         let segments =
             parts
-            |> Seq.map (fun (MultipartItem (formField, fileName, contentStream)) ->
-                let fileExt = Path.GetExtension fileName
-                let contentType = defaultArg (MimeTypes.tryFind fileExt) "application/octet-stream"
+            |> Seq.map (fun (MultipartFileItem(formField, fileName, contentType, contentStream)) ->
                 let printHeader (header, value) = sprintf "%s: %s" header value
 
+                let sharedHeaders = [
+                    prefixedBoundary
+                    HttpRequestHeaders.ContentDisposition("form-data", Some formField, fileName)
+                        |> printHeader ]
+                let headers = match contentType with
+                              | Some(contentType) ->
+                                  sharedHeaders
+                                      |> Seq.append [ HttpRequestHeaders.ContentType contentType |> printHeader ]
+                              | None -> sharedHeaders
                 let headerpart =
-                    [ prefixedBoundary
-                      HttpRequestHeaders.ContentDisposition("form-data", Some formField, Some fileName)
-                      |> printHeader
-                      HttpRequestHeaders.ContentType contentType
-                      |> printHeader ]
+                    headers
                     |> String.concat "\r\n"
 
                 let headerStream =
@@ -2079,7 +2090,17 @@ type Http private () =
                             |> e.GetBytes
 
                         HttpContentTypes.FormValues, (fun e -> new MemoryStream(bytes e) :> _)
-                    | Multipart (boundary, parts) -> HttpContentTypes.Multipart(boundary), writeMultipart boundary parts
+                    | Multipart (boundary, parts) ->
+                        let fileParts = parts |> Seq.map (fun (MultipartItem(formField, fileName, stream)) ->
+                            let fileExt = Path.GetExtension fileName
+                            let contentType = defaultArg (MimeTypes.tryFind fileExt) "application/octet-stream"
+                            MultipartFileItem(formField, Some fileName, Some contentType, stream))
+                        HttpContentTypes.Multipart(boundary), writeMultipart boundary fileParts
+                    | MultipartFormData (boundary, parts) ->
+                        let fileParts = parts |> Seq.map (fun p -> match p with
+                                                                    | FormValue(formField, value) -> MultipartFileItem(formField, None, None, new MemoryStream(Encoding.UTF8.GetBytes(value)))
+                                                                    | FileValue(item) -> item)
+                        HttpContentTypes.Multipart(boundary), writeMultipart boundary fileParts
 
                 // Set default content type if it is not specified by the user
                 let encoding =