Downloadsystem: Detect file-size change and start from scratch
A lot of video plugins have a simple quality selection.
The following is a rare case but would still cause issues with the current download core:
Select quality 240p, add a URL and start downloading.
Then stop the download, select BEST and start it again.
Now the downloadsystem will just continue with the previous file, resulting in a broken files and progress e.g. over 200% until the "real filesize" is downloaded.
Downloadsystem should detect the different content-length and start from the beginning in such a case.
At this moment, only a few plugins will handle this correctly internally e.g. uptobox.com/uptostream.com.