Hi @AnastassiaZ
Remember, the Canvas Inbox is not the same as your email in Outlook, even if you have Canvas Inbox messages send notifications to your Outlook email.
If you are creating a message directly in the Canvas Inbox, you can add an attachment, and the recipient will get the attachment in their Canvas Inbox. If they have notifications about Canvas Inbox messages sent to their email, then the attachment is included with the notification email.
However, if you reply to a Canvas notification about a Canvas Inbox message from your Outlook email, and include an attachment in that reply, that attachment will not be included. Canvas will forward email replies to the Inbox, but it will not forward attachments. If you want to sent and an attachment with a reply, you must reply from the Canvas Inbox, not from your email account.