Detect MS Office temporary files as application/octet-stream

MS Office temporary files are named with an initial _$. For instance a temporary file for myfile.docx will be $myfile.docx.

The files should not be detected by the MimetypeRegistryService as having Office types, but should be considered application/octet-stream, which is the type that Office uses when it PUTs them using WebDAV.

Note that these ~$ files are "Owner Files" and contain information about the Windows user currently editing the file. See "Owner File" in https://support.microsoft.com/en-us/help/211632/description-of-how-word-creates-temporary-files for reference.