The Form upload box will upload the files to the same place files in your Document and Media is stored. By default, Document and Media will store files in {LIFERAY_HOME}/data/document_library. So, if you can't find your file there, than you may have configured your Document and Media to store files in a different location. Check the value of the portal.property 'dl.store.impl' and determine the file store you're using. If you're using one of the file system  store, than go to  System Settings > File Storage to check the path.

HiI created a form and I used an upload box in that.then I uploaded a file via that form.I want to know where is the file that I uploaded?
that isn't there in root\data\document_library 

As Sam said, the data will be in the Document Library. However, if you're looking for a file with the name that you uploaded, you won't find it. It's in storage, in a format that's not meant to be mocked with manually. Don't think of it as File any more. If you're interested in its content, you'll need to find it in the Document Library and access it through the appropriate API.
Document Library can hold many different versions of the same file, and individual sites can all have files with the same name - and they never overwrite each other.

Your information is very interesting. Thank you for sharing
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