iOS 9 has finally brought us the long awaited support for file attachments. Whereas before you could only insert a photo or video, you can now attach spreadsheets, .pdf's, text documents and all manner of other files. To attach a file, simply long press anywhere in the body of the message until a row of options pops up. Hit the right arrow until you see "Add Attachment." Tap on it and it will take you to your iCloud Drive by default. Select a file from here to attach (just tapping on it will attach it), or tap "Locations" in the top right corner to find a file somewhere else like Dropbox, etc.
How do I add an attachment to an email in iOS 9?