application/octet-stream
from
MIME
application/octet-stream
application/octet-stream【MIMEタイプ】とは｜「分かりそう」で「分からない」でも「分かった」気になれるIT用語辞典
application/octet-stream「ファイルの種類は気にするな！」を意図した表現
Unexpected attachment files are triggering Data Protection policy due to MIME type: application/octet-stream
application/octet-stream is the default value for a binary file