Hi Chris,

Try this workaround

1. Insert Two File attachment controls binded to a same field so that they
store the same attachment
2. Also insert a text box below
3. In the first attachment control, Check the check box “Allow the user to
browse, delete and replace files”.
4. Add a conditional formatting to hide this Control when the “text box is
not blank”
5. In the Second attachment control, Uncheck the check box “Allow the user
to browse, delete and replace files”
6. Add a conditional formatting to hide this Control when the “text box is
blank”
7. Now Preview the form and Insert an attachment (Second Control will be
hiding as long as the text box is Blank)
8. Enter some text in the text box
9. Now the second control is shown and it is Read only as per step 5 (First
Control is Hiding  when the text box is not blank)

Hope this helps you.