You can only directly attach photos in the Mail app. They must be in the Photos app. You do so by tapping and holding int he body of the email. When the magnifying glass appears, lift your finger. You'll get a popup menu with the option to Insert Photo or Video.
Most other attachments are done by emailing the document directly from the app that it is stored in. Almost all apps the create documents have an email export option.
Similarly, you can use the Open In option in an app to upload a file to DropBox or another cloud app. You can then use the cloud app to generate a share link and paste that in your email. That will allow your recipient to download the file at their convenience. Some apps even have DropBox (and other services) built in, letting you skip a step or two.
Most of the options I've mentioned tend to be hiding in the share/task icon. It looks like a box with a curved arrow coming out of it. But that is not always true, so you may have to tap around a bit to find what you need.
Good luck.