You have to enumerate the folders in the library, and then from the folder
object call the add  method to add the file to that folder. That was the way
it was done in 2003 (at least the way I did it :) ) I imagine it will still
work that way, if they haven't improved it and allowed you to add the file
to a folder  directly.

When I implemented it I basically parsed the folder path where I wanted the
file to be uploaded to, and checked that each leaf of the folder existed, if
it didn't I'd create it and continue from there until I had the entire
folder path built, then I would just add the file to this final folder
object.

I know how to add  a file to a folder  once
I can identify the folder. The question here is how to identify the
folder in my custom action. How did you get your folder path?

I know which folder  it's going to, it's not happening with the event handler
like it is in yours.

If you want to know where the file is located, you'll need to look at the
itemURL and parse it to remove the site name, and the root  url, then you'll
have the folder path for the file.

The itemURL is not populated with any value. Here is the URLAction
value: <UrlAction
Url="~site/_layouts/MWE/InterwovenUpload.aspx?ListId={ListId}&ItemId={ItemId\
}&ItemUrl={ItemUrl}"/>

Neither {ItemUrl} nor {ItemId} is populated with value.

You need to look at the URL of the item  you just uploaded, not what's in the
URLAction.

I am confused here. At this point of the process, the
item/document is not uploaded yet, so there is no URL to look at.
Maybe I missed something here.

There is another approach I can try, which uses web services, if I
can't figure out how to upload  a document  to a subfolder by the
approach I described earlier.