If the attachment from the end user does not appear on zendesk, the first possibility is that the attachment exceeds the maximum capacity. For zendesk's Enterprise plan, the maximum file size for attachments is 20MB.
If you are going through an external mailer and the size of the attachment is larger than Zendesk, even if the attachment is displayed in the mail account, it will not be displayed when the file is transferred to Zendesk due to the capacity limit. For example, Gmail can attach files up to 25 MG, so even if you can check it on Gmail, you may not be able to see the attached file when transferred to Zendesk.
In this case, it is necessary to ask the end user to send again a file in a smaller size, but if you use a little trick to open the attached file that exceeds the size limit, you can open the file that exceeds the size limit (See How to get around Zendesk's attachment size limit). However, it will take time if you use this trick every time, so we recommend that you inform the end user in advance of the size limit and have them send files within the capacity.