ファイルのMimeを判断する

ファイルがテキストファイルなのか、バイナリファイルなのか…判断したい場合、FindMimeFromDataが使えます。
FindMimeFromDataは、データを解析し、以下のMimeを判断できるようです。

◆FindMimeFromDataが返すMimeリスト

MIME Type	Description
text/plain	Plain text. Default if data is primarily text and no other type detected.
text/html	HTML. Default if common tags detected and server does not supply image/* type.
text/xml	XML data. Default if data specifies
text/richtext	Rich Text Format (RTF).
text/scriptlet	Microsoft Windows script component.
audio/x-aiff	Audio Interchange File, Macintosh.
audio/basic	Audio file, UNIX.
audio/mid	Internet Explorer 7 and later. MIDI sequence.
audio/wav	Pulse Code Modulation (PCM) Wave audio, Windows.
image/gif	Graphics Interchange Format (GIF).
image/jpeg	JPEG image.
image/pjpeg	Default type for JPEG images.
image/png	Internet Explorer 7 and later. Portable Network Graphics (PNG).
image/x-png	Internet Explorer 7 and later. Default type for PNG images.
image/tiff	Tagged Image File Format (TIFF) image.
image/bmp	Bitmap (BMP) image.
image/x-xbitmap	Removed from Internet Explorer 8.
image/x-jg	AOL Johnson-Grace compressed file.
image/x-emf	Enhanced Metafile (EMF).
image/x-wmf	Windows Metafile Format (WMF).
video/avi	Audio-Video Interleaved (AVI) file.
video/mpeg	MPEG stream file.
application/octet-stream	Binary file. Default if data is primarily binary.
application/postscript	PostScript (.ai, .eps, or .ps) file.
application/base64	Base64-encoded bytes.
application/macbinhex40	BinHex for Macintosh.
application/pdf	Portable Document Format (PDF).
application/xml	XML data. Must be server-supplied. See also "text/xml" type.
application/atom+xml	Internet Explorer 7 and later. Atom Syndication Format feed.
application/rss+xml	Internet Explorer 7 and later. Really Simple Syndication (RSS) feed.
application/x-compressed	UNIX tar file, Gzipped.
application/x-zip-compressed	Compressed archive file.
application/x-gzip-compressed	Gzip compressed archive file.
application/java	Java applet.
application/x-msdownload	Executable (.exe or .dll) file.

http://msdn.microsoft.com/en-us/library/ms775147(VS.85).aspx

◆サンプルソース

<DllImport("urlmon.dll", CharSet:=CharSet.Auto)> _
Private Function FindMimeFromData( _
    ByVal pBC As IntPtr, _
    <MarshalAs(UnmanagedType.LPWStr)> ByVal pwzUrl As String, _
    <MarshalAs(UnmanagedType.LPArray, ArraySubType:=UnmanagedType.I1, SizeParamIndex:=3)> ByVal pBuffer As Byte(), _
    ByVal cbSize As Integer, _
    <MarshalAs(UnmanagedType.LPWStr)> ByVal pwzMimeProposed As String, _
    ByVal dwMimeFlags As Integer, _
    ByRef ppwzMimeOut As IntPtr, _
    ByVal dwReserved As Integer) As Integer
End Function

Public Function getMimeFromFile(ByVal file As String) As String

    Dim mimeout As IntPtr

    If Not System.IO.File.Exists(file) Then
        Throw New FileNotFoundException()
    End If

    Dim MaxContent As Integer = CInt(New FileInfo(file).Length)

    If MaxContent > 256 Then MaxContent = 256

    Using fs As New FileStream(file, FileMode.Open)
        Dim buf(MaxContent) As Byte
        fs.Read(buf, 0, MaxContent)
        fs.Close()
        Dim result As Integer = FindMimeFromData(IntPtr.Zero, file, buf, MaxContent, Nothing, 0, mimeout, 0)
    End Using

    Dim mime As String = Marshal.PtrToStringUni(mimeout)

    Marshal.FreeCoTaskMem(mimeout)

    Return mime
End Function

http://dobon.net/cgi-bin/vbbbs/srch.cgi?PAGE=20&andor=and&logs=.%2Fvb%2Fvb.dat&no=0&word=intptr

◆補足

getMimeFromFile関数の引数へファイルパス名を指定します。リターン値は、MIME Typeです。

尚、マーシャル（Marshal）は、あるプログラムで利用されるデータ形式を、異なるプログラムで利用可能な形式に変換することです。

MarshalAsやMarshalでＣ言語のデータ形式をVBで利用可能に変換しているようです。

◆留意点

ご自分が使うシステムで念入りに検証してから適用した方が良いでしょう！

◆参考

• http://msdn.microsoft.com/en-us/library/ms775107(VS.85).aspx
• http://msdn.microsoft.com/en-us/library/ms775147(VS.85).aspx