Just thought I'd mention that if you're trying to use the Upload class with mime_whitelist, Office 2007 files (those with extensions like docx, xlsx, etc.) are identified as mime type "application/zip" instead of "application/vnd.openxmlformats-officedocument.wordprocessingml.document" and
"application/vnd.openxmlformats-officedocument.spreadsheetml.sheet." This makes some sense because they are essentially zip files.
My workaround is to also white list mime type "application/zip." Unless someone can think of a better solution.