Unable to add Schemas (xsd) in XML setting

I am trying to add the xsd file (schema) for the XML in IntelliJ but when I select any file, the add button gets disabled.

For adding schema, I followed following path:

File -> Settings -> Languages & Frameworks -> Schemas and DTDs - (+) button under 'External Schemas and DTDs' section

Please find the attached image below.

Thanks in advance for the help :)