It would be more convenient to new users if a "category" field was added on the
upload form, at least on Commons where categories are essential for files.
This field could be similar to the one found in the tool Commonist
(http://djini.de/software/commonist/), where category names are entered,
separated with a vertical bar.
The categories would simply be added to the wikitext, like the license dropdown
selector does for the license bit.