Just to add to this thread - whenever you get this problem it's nearly always the fact that the actual file size is different from the file size in the 'Files' table.  This can sometimes by fixed with a 'sync' but not necessarily. 

You can check this by doing a select * from files to find your specific file, and compare the file size with the file on your computer (note, you need an accurate byte size, not '1kb'.  The bytes of the file and the bytes in the Files table must match exactly, because the fileserverhandler in DNN will send the download size with the request, and it gets this from the table, not the actual file.

Sometimes the file details have been cached by DNN, and multiple attempts to sync this will not fix it.

When this has persisently happened to me, I manually update the table with the correct file size and restart the application.  That's my brute-force, fallback method for fixing this problem when it happens.  I don't recommend it as a daily process but if you're stuck it might help.