MimeTip.fs

#if INTERACTIVE
#r "System.Net.Http"
#endif

module MimeTip

open System.IO
open System.Net.Http
open System.Runtime.InteropServices

[<DllImport("urlmon.dll", CharSet = CharSet.Auto)>]
extern uint32 private FindMimeFromData(
    uint32 pBC,
    [<MarshalAs(UnmanagedType.LPStr)>] string pwzUrl,
    [<MarshalAs(UnmanagedType.LPArray)>] byte[] pBuffer,
    uint32 cbSize,
    [<MarshalAs(UnmanagedType.LPStr)>] string pwzMimeProposed,
    uint32 dwMimeFlags,
    [<Out>] uint32& ppwzMimeOut,
    uint32 dwReserved)

let smime (stream: Stream) =
    let buffer = Array.zeroCreate 256
    ignore (stream.Read(buffer, 0, 256))
    try
        let mutable mimetype = 0ul
        match FindMimeFromData(0ul, null, buffer, 256ul, null, 0ul, &mimetype, 0ul) with
        | 0ul -> let mimeTypePtr = nativeint mimetype
                 let mime = Marshal.PtrToStringUni mimeTypePtr
                 Marshal.FreeCoTaskMem(mimeTypePtr)
                 Some mime
        | _ -> None
    with _ -> None

let asyncwmime url = async {
    use client = new HttpClient()
    use! stream = client.GetStreamAsync(url: string) |> Async.AwaitTask
    return smime stream
}

let wmime url = asyncwmime url |> Async.RunSynchronously

let mime filename =
    use stream = File.OpenRead filename
    smime stream

usage:

open MimeTip
mime @"<path>\file.png"
mime @"<path>\file.html"
mime @"<path>\file.jpg"
wmime "http://<path>/file.png"