Cannot Implicitly Convert Type 'System.Web.HttpPostedFile' To 'System.Web.HttpPostedFileBase'?

I have this method which does not build, it errors with the message:

Cannot implicitly convert type 'System.Web.HttpPostedFile' to 'System.Web.HttpPostedFileBase'
I really need this to be of type HttpPostedFileBase instead of HttpPostedFile, I have tried boxing and it does not work:
foreach (string inputTagName in HttpContext.Current.Request.Files)
{
HttpPostedFileBase filebase =HttpContext.Current.Request.Files[inputTagName];
if (filebase.ContentLength > 0)
{
if (filebase.ContentType.Contains("image/"))
{
SaveNonAutoExtractedThumbnails(doc, filebase);
}
}
}