No, you can't change them. The attachments field is the way it is because it needs to be (changing it to text would be pointless as it wouldn't be able to handle attachments any more).
The simple solution for that question is to disallow attachments and add a custom text field.
No, We cannot change the built-in types until and unless you hack the jira code. That is the complex solution.
Instead if you need Text Field itself, there exists a Text Field built-in type.
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