Hi Pranay, can you please provide more details on what application you want to disable the ability to download email attachments. 

Outlook

I'm looking in the Preference keys at Microsoft https://docs.microsoft.com/en-us/deployoffice/mac/preferences-outlook as well as this google sheet https://docs.google.com/spreadsheets/d/1ESX5td0y0OP3jdzZ-C2SItm-TUi-iA_bcHCBvaoCumw/edit#gid=0 that may control the ability to disable downloading attachments. But, I don't see it. If you are using Office 365. You should be able to restrict it from the admin console for Office. 

I'm using Office 365 , How do I do it from admin console?

I think the conditional access policy only applies to the OWA, not the actual desktop client. You can read more here. https://petri.com/conditional-access-blocks-office-365-downloads

Here I can Suggest a Tool that can help to extract attachments from emails for Windows operating systems if you think it is useful then should try the Softaken Gmail Attachment Extractor Tool it is the best solution to extract attachments from a Gmail account email should try it because I tried this application this last week and I get the best result with this. Through this utility users can extract PDF, DOC, and XLS format attachments easily and with this tool you can download bulk emails without any technical errors. Install this app on all versions of Windows OS.

More Info: https://www.softakensoftware.com/gmail-attachment-extractor.html