TYPO3 Core

The Use of 'EXT:<filePath>' is common for declaring al filepath on various place in TYPO3 and in the TCA. (IconAPI, ...) Only in the definition of DS, it is uncommen.
Only for the filedefinition of Flexforms you have to use 'FILE:EXT:'.
If you use EXT: it is clear, that the string will points to an file. The information FILE: is not needed.

See Patch

Additionl:
It took me more than an hour, to understand my mistake. The exceptionmessage was meaningless. Please change it.

I mean the exceptionmessage in typo3\sysext\backend\Classes\Form\FormDataProvider\TcaFlexProcess.php in line 48.
Instead of

                throw new \RuntimeException(
                    'Data structure identifier must be set, typically by executing TcaFlexPrepare data provider before',
                    1480765571
                );

                throw new \RuntimeException(
                    'Data structure identifier must be set, typically by executing TcaFlexPrepare data provider before. Check, if the filepath in the DS-field begins with FIILE: or EXT:',
                    1480765571
                );