Fix for EZP-20741: Allow default content disposition to be overridden

We have clients who want to set the default content disposition to "inline" for all MIME types. This is hard-coded to "attachment" and you have to set the content disposition to "inline" for each MIME type – this is difficult because you have to build a list of every MIME type and maintain this list for future MIME types.
If you are able to set the default content disposition for all MIME types to be "inline" then you can essentially defer to browser behavior: if it can open a document inline, then it does. Otherwise, it falls back to "attachment" anyway.