[TYPO3] Hook when saving content element in BE

[Matthias Stuebner]

Hello all,

because I just don't know where and what to search for:

What hook (or similar) can I use, if I want to verify/modify data entered
into a content elements BE-Form before it is saved?

Detailed example: If the user uploads a file as part of the content element
I want to fill the file size of that file into a db field automatically
instead of letting the user enter the data. But how to hook in?

And as add on: How do I define which data of a content element is shown in
the summary when WEB/PAGE is selected, i.e. in the list of content elements
in column normal?

Thanx in advance,
-- 
br Matthias