Greetings for the day. Hope you doing good.
We have come across this issue today when we installed a custom field plugin for Virtuemart. It created a new custom field type "file". We created custom fields of that type but those are not appearing in field name dropdown in Fields Configuration. We have checked the database table '#__virtuemart_customs' and we can see the fields there which we created (field_type "E"

Vituemart Version: 2.9.9.3
CSVI Version: 5.20
Plugin Link: http://www.wdmtech.com/hexsys-virtuemart-product-attachment-plugin
Please check.
Thanks and regards
Convergence Team