Whatever the images you're posting are, they don't display on this forum and, hence, I can't see what they're supposed to display. Thus I have no idea what the messages in posts 1 & 3 are.

A file with a line like:
Content-type: application/vnd.ms-excel
simply tells a web browser to treat the content as an Excel file. For Excel 2007 & later xlsx files, so too does:
Content-type: application/vnd.openxmlformats-officedocument.spreadsheetml.sheet

That said, unless the file is being named by Excel, I doubt there's anything that can be done about that from the Office side of things; it's entirely under to control of the software that's creating the files.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]