When sending an email via automation/email campaign/one-on-one, files can be added to the email as an attachment or as a link to said file.
But when a user replies to an email (to build on the same email thread), the Attach File option does not come. This means that files can only be sent as a link and not an attachment.
There is a workaround to this. In the URL of the tab, add “&allowAttachments=true”. Once done attachments can be added to the emails in the reply function also.