You can use the MimeType property of the IIsMimeType object to GET and PUT the MIME types for objects of type IISMimeType.
DirObj.MimeType=string
<%
Dim DirObj
Dim MimeMapNode
Dim MimeMapList
Dim MimeMapEntry
Set Obj = GetObject("IIS://LH/MimeMap")
MimeMapList = DirObj.MimeMap
Redim preserve MimeMapList (Ubound(MimeMapList)+1)
Set MimeMapEntry = CreateObject ("IIsMimeTypeEntry")
MimeMapEntry.MimeType = "Text/Plain"
Mime.Extension = ".log"
Set MimeMapList (Ubound(MimeMapList)) = MimeMapEntry
DirObj.MimeMap = MimeMapList
DirObj.Setinfo
%>