Gmail detects these phrases to remind you to actually attach the file if you haven’t:

• see attached
• see attachment
• see included
• is attached
• attached is
• are attached
• attached are
• attached to this email
• attached to this message
• I’m attaching
• I am attaching
• I’ve attached
• I have attached
• I attach
• I attached
• find attached
• find the attached
• find included
• find the included
• attached file
• see the attached
• see attachments
• attached files
• see the attachment