Gmail has a built-in feature that reminds you of adding file attachments before the email is sent to the recipient. Gmail scans the text of your email and shows an alert if it thinks you intended to include a file attachment in the message but forgot to do so.

If you haven’t seen this earlier, compose a new email in Gmail, add “I’ve attached the file” in the body and hit send. Gmail will pop-up a warning saying - “it seems like you forgot to attach the file.”

How does the forgotten attachment detector work inside Gmail? I was looking at the source code of Gmail.com using Chrome Dev Tools and found an interesting snippet that bares it all.

When you hit the send button, Gmail scans the text of your outgoing message and looks for the following word combinations using regex.

|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

If a match is found, a warning is shown so you’ll never forget to include that important attachment again.

Microsoft Outlook too includes an attachment detector feature and, unlike Gmail, it also provides you an option to define custom keywords that should trigger the missing attachment alert.

Also see: Download Gmail Attachments to Google Drive