Hi @Kayhan Kaya ,
Thanks for your reply!
The gmail account is not listed on web Outlook at all.
This is not for you to check the web version of Outlook, just to check if the attachment in the web version of the mailbox exists and can be opened normally. Just log in to the Gmail.com to check.
On the other hand, did you encounter the issue is New Outlook? If so, i found a known issue mentioned that that is may be related to macOS Big Sur. It's suggested that you could check the macOS Big Sur version.
More information: Attachments missing from emails when using macOS Big Sur.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.