When I save a docx or a xlsx file with Libre office, the mimetype of these files are "application/octet-stream" and when uploading them as attachments - they get renamed to .txt which is not desired behavior.

I've tracked this bug down to "media/k2/assets/vendors/verot/class.upload.php/src/class.upload.php" where there is only one mimetype defined for each of the files, but as you see - they can come as both.

I've managed to fix this by adding an additional "mime_types_extra" property to the class and doing an extra "else if" check on line 2987 to check this other array for mime types.

If you like, I can make a pull request.

Here are the test files saved with Libreoffice
attachment.docx
attachment.xlsx