If a URL has an expiration time, or if one simply wishes to download the same file from another location due to speed constraints on one server, or other reasons, then there is no current possibility to continue the download.

Whenever a second file (with the same name) is passed, KGet asks if we want to restart the download. There should be an option to "resume" or "continue" the download, in addition to simply restarting.

This should work in two ways:

1. If a download already exists in KGet (but stopped), it should continue the download as per normal, with all settings including multipart (the resume should also be multipart).

2. However, if a download is new in the list but the file already exists on the filesystem, multipart should be disabled for that resume, so as to not corrupt the file in the event the partial file was a leftover of another multipart download manager.

If the file was downloaded with another download manager, it will not be KGet's fault if it was part of a threaded (multipart) download. The resulting file will simply be corrupted in that case. In other words, no download manager worth its salt should leave a partial multipart file as if it was a normal download (it should be split into many files).