Yes it is annoying, here is how to solve it (Paul should also make this small change to the editor for future versions):
1. Open "Tier 1\Editor\Data\tags\main.agc.tags" in a text editor such as Notepad.
2. Insert the following line into the top of the file on its own line:
String|String|(String Data Type)|
3. Save the file.
4. Reopen the AppGameKit editor.
Now you can enter "As String" and it will both autocomplete to "String" and allow you to still autocomplate to the Base64 version as needed but it won't auto-switch it.