@Janani Sorry about the delay here and in the related thread you had posted. We are waiting to hear back from the freshsales suite team. I have nudged them again, and I will update you as soon as I hear something.
@Janani Just heard back from the freshsales suite team. Unfortunately, there are no public APIs today to add attachments to Custom Modules.
If you are building an app, there might be alternative solutions to achieve what you need. This will involve storing the attachments in an external service and maintaining a mapping between records. The exact implementation will depend on the details of your use case.