Plugin writing: access file that was just uploaded

When you use media_handle_upload() to upload a file, it creates the attachment post in the database and return the ID of the attachment, or a WP_Error if the upload failed. This ID of the attachment is used to access uploaded file.
See documentation here. and an example here.

So path to your CSV file can be retrieved using get_attached_file() as follows:

$my_file = get_attached_file( $attachment_id ); // Full path

Documentation for get_attached_file() is here.
I hope this may help.

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