A file extension of .download is not a standard file that I'm aware of. Is it related to Zimbra in some way? How you would open the file would depend you the operating system you are using, and what type of file it is.
The best suggestion for how to answer your question, for file extentions more normal than this one at least, is to use google.
I just tried that myself, using the following URI
how do i open .download files - Google Search
lo and behold the first answer may be the one you are looking for, do you use P2P applications at all?
Bugzilla
Wiki
Source
*.download help 
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