How do I open files from other apps?
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.
Good Reader
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
ReaddleDocs
Use the Open in... command from the toolbar in the document preview.