[BUGFIX] impexp: File upload in import broken

Uploading import files broke with patch for issue #81763
Fix by renaming POST array back from 'data' to 'file'.

Change-Id: I8de880775e2949951ef8a3f45b0a704c2ef3ac76
Resolves: #83599
Related: #81763
Releases: master
Reviewed-on: https://review.typo3.org/55385


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>