I want to allow upload excel if there are specific fields in the file.

Unfortunately, Jotform can't read what's inside the attached file. What you can probably do is put a field that asks a question to your customers and if certain question is selected, you show a file upload field that allows them to attach an excel file.

ex.

Guide: https://www.jotform.com/help/316-how-to-show-or-hide-fields-base-on-users-answer