[TYPO3-mvc] image upload an description

[Christian Tauscher]

Hello again.

I have sucessfully implemented a file upload based on Helmut Hummels 
upload_example to my own extention.

Next for me is to implement a "description" line, so with every image a 
"description" can be uploaded.

My Problem is:

in The Form: if I introduce a caption field (input), how do I "map" this 
to the image, and how can this field be prefilled in case of 
edit-action, since it is not a real field in my model by of the 
sys_file_reference.

Do you know a Example EXT where I can see how the world works?

Christian.