The addon has no concept of HEAD and GET. It can simply read from the APIs that kodi provides. This question appears to me more about those APIs.
Then let me rephrase this. Does the Kodi API provide the return values of Kodi's HEAD request?
If no, then there's nothing you can do. If yes, then the next question is: Does your addon utilize that API endpoint?
If no, then why not? If yes, then there must be a bug. Because without a KODIPROP the log says inputstream is detecting "application/octet-stream" as a mimetype.
The is only one endpoint which is to get the content of a file. There is no concept of a head request.
It simply opens a file.