TYPO3 Core

There's the possiblilty to limit the file upload in the backend to certain file extensions. For example
<config>
<type>group</type>
<internal_type>file</internal_type>
<allowed>gif,jpg,jpeg,tif,bmp,pcx,tga,png</allowed>
</config>

According to the example in TYPO3 core API documentation ( http://typo3.org/documentation/document-library/core-documentation/doc_core_api/current/view/4/2/#id2817038 ), one could use $GLOBALS['TYPO3_CONF_VARS']['GFX']['imagefile_ext'] instead of the comma-seperated list of extension.

This works in tca.php but not in flexform_ds.xml

(issue imported from #M4511)