Qftp: After the downloading cancelled, the file can't be re-downloaded.

How to reproduce:
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1. Individually launch qftp application.
2. Tap the "Connect" button and wait for connecting and loading finished.
3. Tap the folder list and navigate to a detail file list and download a file.
4. During downloading, tap the "Cancel" button.
5. The "Download" button turns invalid, tap another file, it turns back to valid.
6. Tap the original file again, the "Download" button turns valid.
7. Tap "Download" button.

Expected result:
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The re-downloading should be permitted.

Actual result:
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Message shown that "There already exists a file...", actually the file isn't downloaded completely because of the cancellation.

More detail in private JIRA: https://qtrequirements.troll.no/browse/QT-4836