Currently, all scheduled reports are sent as content in the main body of the report's email.
To request that this feature be included in a future Salesforce release, please upvote this idea: https://success.salesforce.com/ideaView?id=08730000000Brb6AAC
As a workaround, you can achieve this functionality with programming.
This link [1] has all the info you need.
Regards.
[1]
https://help.salesforce.com/HTViewHelpDoc?id=reports_schedule.htm&language=en_US
To request that this feature be included in a future Salesforce release, please upvote this idea: https://success.salesforce.com/ideaView?id=08730000000Brb6AAC
As a workaround, you can achieve this functionality with programming.