Many iPad apps support the Open In... menu. See some examples.
If you have an email with plain text attachments, simply tap and hold it. (1)
Use the Open In Textastic (2) command to open this attachment in Textastic.
1) Open the Manage Files panel.
2) Select the file you want to open with Textastic.
3) Tap on the Open In... button
4) Choose Textastic