By default, Outlook Web App blocks attachments that have the following file name extensions: This issue occurs if the attachment is blocked by Outlook Web App. Instead, a generic placeholder, such as '1 Attachment' is displayed, and this can't be opened.
in Microsoft 365, the attachment isn't displayed in the message.When Microsoft 365 users try to open or view attachments in email messages in Outlook Web App, they experience the following symptoms: