You can upload multiple files from a single <s:file> element with multiple="multiple" like described here.
You can also upload multiple files from many <s:file> elements (that allow a single file for each one) in the same way, handling the names of the <s:file>s to point to a list on the Action.
Do you really want to upload a Lists of Lists of Files ?
If yes, I suggest you to model an object, like MyFileListObject, containing the lists of data needed:
class MyFileListObject {
private List<File> files;
private List<String> filesContentType;
private List<String> filesFileName;
/* getters and setters */
}
and then expose a List<MyFileListObject> through the Action.
Alternatively, you can granulate it more, defining a new object, like MyFileObject,
class MyFileObject {
private File files;
private String filesContentType;
private String filesFileName;
/* getters and setters */
}
,listed in MyFileListObject:
class MyFileListObject {
private List<MyFileObject> files;
/* getter and setter */
}
and then expose a List<MyFileListObject> through the Action.
But it seems overkill to me... which kind of page should let many <input type="file"/> upload many files each one in a single post ?