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- You can try creating a torrent with the file(s) you have at hand and try to search with the info hash. You need to be sure the metadata (e.g. name, size, directory stucture, etc.) of the file(s) in question is identical to the original download.
- You can try adding the torrent into a client. Pause it and then copy the file(s) to the download destination. Rename the file(s) accordingly. Ask your client to recheck/rehash the file(s). If the file(s) is/are identical, it will work.