In iOS9 you tap on a blank area of the message then select 'add attachment' to browse for the file. It supports iCloud Drive, DropBox, OneDrive, and presumably

In iOS9 you tap on a blank area of the message then select 'add attachment' to browse for the file. It supports iCloud Drive, DropBox, OneDrive, and presumably any other apps that support the cloud storage APIs introduced last year in iOS8. Alternatively you can use the traditional method of exporting a file from an app into Mail.