FWIW I sent myself a xlsx file with the same name as yours. It came in with the same Content-Type (application/vnd.openxmlformats-officedocument.spreadsheetml.sheet) and MIME::Parser saved the attached file correctly. So I don't think Content-Type is the problem. Could it be some type of filename length restriction - try adding something like this to your script:

system('getconf NAME_MAX ' . $tmpdir);
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