On file upload when file exists ask for new name

[laoneo]

trafficstars

Actually when a file exists the user has the option to overwrite it or cancel the action. As discussed in #184 it would be nice to offer the user the possibility to give the file a new name. We need to keep in mind what we do when multiple files are uploaded.

[dneukirchen]

Alternatively we can add an incrementing Suffix. my_file(1).jpg, my_file(2).jpg ...