Can the bot reply with an attachment?

Yes. Please refer to the tutorial Create a dialog flow - There are five different kinds of replies a bot can make.

Can I fill multiple slots in one sentence? For example, “I want five pounds of apples”

Yes. You can define multiple slots in a form, and we will automatically recognize multiple slots. For more details, please see here.

During a conversation, I want to output different responses according to different user inputs. What should I do?

There are several ways to do it.

Solution 1: Branching a dialog flow, and add a condition in each branch. The condition could be whether a slot value is null. Please refer to Bot Reply Conditions.

Solution 2: Different slot values trigger different responses. Please refer to Conditional Responses. This solution can reduce the number of branches displayed in your flow.

Solution 3: Calling Webhook. The response is dynamically generated by the value retrieved by calling webhook. For example, query the balance of an account.