More than One Solution: reply all with attachments add in for outlook
You can manually fix this issue by simply copying the original attachment and pasting it in the reply email you are sending.
Click the email containing the attachment.
Right click on the attachment, and click on ‘Select All’.
Now your attachment or attachments will be selected, right click on them, and select ‘Copy’.
Hit reply in the email message.
Now in the reply email message in Outlook right click and select ‘Paste’, to attach your files to your replying email.