Index: web/typo3/sysext/core/Classes/Configuration/FlexForm/FlexFormTools.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/web/typo3/sysext/core/Classes/Configuration/FlexForm/FlexFormTools.php b/web/typo3/sysext/core/Classes/Configuration/FlexForm/FlexFormTools.php
--- a/web/typo3/sysext/core/Classes/Configuration/FlexForm/FlexFormTools.php	(date 1626090979318)
+++ b/web/typo3/sysext/core/Classes/Configuration/FlexForm/FlexFormTools.php	(date 1626090979318)
@@ -682,15 +682,17 @@
         // Hooks may have parse the data structure already to an array. If that is not the case, parse it now.
         if (is_string($dataStructure)) {
             // Resolve FILE: prefix pointing to a DS in a file
-            if (strpos(trim($dataStructure), 'FILE:') === 0) {
-                $file = GeneralUtility::getFileAbsFileName(substr(trim($dataStructure), 5));
-                if (empty($file) || !@is_file($file)) {
-                    throw new \RuntimeException(
-                        'Data structure file ' . $file . ' could not be resolved to an existing file',
-                        1478105826
-                    );
-                }
-                $dataStructure = (string)file_get_contents($file);
+            foreach(['EXT:'=>0, 'FILE:'=>5] as $check => $length ) {
+                if (strpos(trim($dataStructure), $check) === 0) {
+                    $file = GeneralUtility::getFileAbsFileName(substr(trim($dataStructure), $length));
+                    if (empty($file) || !@is_file($file)) {
+                        throw new \RuntimeException(
+                            'Data structure file ' . $file . ' could not be resolved to an existing file',
+                            1478105826
+                        );
+                    }
+                    $dataStructure = (string)file_get_contents($file);
+                }
             }
 
             // Parse main structure