[TYPO3] Error when editing pagecontent that is an extensions

[Rolf Kutz]

Hello,

I sometimes get this error "You have chosen to open
alt_doc.php which is a php-script[...] What should Firefox
do with this File?[...]" when trying to edit a
page content that is an extension. This happens
with "cal" i.e."

What causes this and how can I fix it?

regards, Rolf