Update attachment-extension.cds for Drop down

[PujaDeshmukh17]

Describe your changes

This PR will add the changes required to the leading application to encorporate the drop down input type

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist before requesting a review

• I follow Java Development Guidelines for SAP
• I have tested the functionality on my cloud environment.
• I have provided sufficient automated/ unit tests for the code.
• I have increased or maintained the test coverage.
• I have ran integration tests on my cloud environment.
• I have validated blackduck portal for any vulnerability after my commit.

Upload Screenshots/lists of the scenarios tested

• I have Uploaded Screenshots or added lists of the scenarios tested in description