Hi @p_v_nair ,
There is no direct OOTB way of restricting upload of assets without an extension, but you could achieve this with few configurations/customisations as below:
1. AEM by default identifies the mime type of an asset using its extension. For assets without an extension, there will not be any mime type detected. So, to restrict upload of these kind of assets that don't have a detected mime type, Disable the "Allow All Mime" Flag of DAY CQ Asset Upload Restriction configuration. But, while doing so, please make sure that you are adding all the expected different Mime types in the "Allowed Asset MIMEs" field.
2. Customise the DAM Update Asset Workflow with an additional process step that checks for extension less assets and deletes them.
Hope this helps.
Thanks!