add validation for the intended save file type.

Make "drawing" commands async.
Minor spelling changes (comments).

DeathSocket/GridPainter.fs

@@ -31,7 +31,8 @@ namespace DeathSocket
         let applyGridAsync spec =
             async {
                 try
-                    validateFilePath |> ignore
+                    validateFilePath spec.originalPath |> ignore
+                    validatFileType spec.savePath |> ignore
                     drawGrid spec |> ignore
                 with
                 | :? FileNotFoundException as ex ->

DeathSocket/ImageServices.fs

@@ -3,7 +3,6 @@
     open System.Drawing
     open System.Drawing.Imaging
     open DeathSocket
-    open System.Drawing.Drawing2D
 
     let createHorizontalLines width height rows =
         let interval = height / rows
@@ -18,7 +17,7 @@
                Point ((interval * point), height)|]|]
 
     let drawGrid spec =
-        // The temp. file is used as a way to convert images with index pixels.
+        // The temp. file is used as a way to convert images with indexed pixels.
         use original = Bitmap.FromFile spec.originalPath
         use temp = new Bitmap(original)
         use clone = temp.Clone(new Rectangle(0, 0, temp.Width, temp.Height), PixelFormat.Format32bppArgb)

DeathSocket/Validation.fs

@@ -1,9 +1,20 @@
 module internal Validation
 
-open System.IO
+    open System.IO
 
     let validateFilePath path =
         match File.Exists path with
         | true -> ()
         | false -> raise (new FileNotFoundException (path + " could not be found."))
 
+    let validatFileType file =
+        match Path.GetExtension file with
+        | ".bmp" -> ()
+        | ".BMP" -> ()
+        | ".jpg" -> ()
+        | ".JPG" -> ()
+        | ".png" -> ()
+        | ".PNG" -> ()
+        | ".tif" -> ()
+        | ".TIF" -> ()
+        | _ -> invalidArg "savePath" "The file type must be a .bmp, .jpg, .png or .tif file."

DeathSocketCLI/Commands.fs

@@ -39,7 +39,7 @@
                 printfn "[INFO.] Adding default grid to image..."
                 buildDefaultSpec imgPath newPath
                 |> applyGridAsync
-                |> Async.RunSynchronously
+                |> Async.Start
                 showEndOfCommandMessage
             with
             | :? FileNotFoundException as ex -> "[ERROR] No file was found at " + ex.FileName
@@ -58,7 +58,7 @@
                 printfn "[INFO.] Adding grid to image..."
                 buildSpec imgPath numRows numColumns pWidth colour newPath
                 |> applyGridAsync
-                |> Async.RunSynchronously
+                |> Async.Start
                 showEndOfCommandMessage
             with
             | :? FileNotFoundException as ex -> "[ERROR] No file was found at " + ex.FileName