How to update media file (.csv) only using curl? - #2 by LN describes how. @KitoDavy, I see you never got a response to your follow-up question there which was the same as @varilla's original one here -- you can update just the media by sending the same blank form.
The full curl command would be something like:
curl --verbose --digest --user <username>:<password>
--header "Content-Type: multipart/form-data" \
--form form_def_file=@my-form.xml \
--form mediaFiles=@/Users/data.csv \
--form mediaFiles=@/Users/image1.jpg \
--form mediaFiles=@/Users/image2.jpg \
https://<servername>.appspot.com/formUpload
Note that what you're doing in that case is filling out the web form as though you were a browser. There's no nice API for doing these kinds of things in Aggregate which is one of the reasons ODK Central is being developed.