checks the extension of a file name

isimage.fs

type FileExtension = 
        |PNG
        |JPG
        |GIF
        |RAW
        |BMP
        |SVG
        |PSD

    type IsImageType = 
        | IsAnImage
        | IsNotAnImage
    
    let (|PNG|JPG|GIF|RAW|BMP|SVG|PSD|) (input : string) = 
        let fileType = input.Split '.' |> Array.last
        match fileType.ToLower() with
        | "psd" -> PSD
        | "svg" -> SVG
        | "bmp" -> BMP
        | "raw" -> RAW
        | "gif" -> GIF
        | "jpg" -> JPG
        | "png" -> PNG
        | _ -> failwith "Unknown file type"
    
    let IsImageUrl str = 
        match str with
        |PNG |JPG |GIF|RAW|BMP|SVG|PSD -> IsAnImage
        | _ -> IsNotAnImage