18.104.22.168.2.1 File Name
For the file name in a MIME representation of an attached file, MIME writers SHOULD use the value of the PidTagAttachLongFilename property ([MS-OXCMSG] section 22.214.171.124). If this value is not available, MIME writers SHOULD use the value of the PidTagAttachFilename property ([MS-OXCMSG] section 126.96.36.199). If the value of the PidTagAttachFilename property is also not available, MIME writers SHOULD use a default value or MAY use an empty string.<82> The attached file name SHOULD be written to several different MIME headers (2), as specified in section 188.8.131.52.2.2.
If a file extension is needed for mapping the attachment content type, it SHOULD be obtained by copying all characters after the last "." (U+002E) character in the file name.