If you open the official messaging App, go to menu then settings, there is a setting "creation mode" which has the following options (in that order):
What does this change? Is this documented somewhere?
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• MMS creation mode — Select Restricted to have your device prevent you from including content in multimedia messages that may not be supported by the network or the receiving device. To receive warnings about including such content, select Guided. To create a multimedia message with no restrictions on attachment type, select Free. If you select Restricted, creating multimedia presentations is not possible.
Messages longer than 3 texts in length get sent as MMS (picture messages) rather than SMS (texts), thereby costing more than texts, or being outside your contract (if you have pre-paid texts).