You can always get a false positive with mime magic though... We can never be completely certain, so I tend to think the line should be drawn at "generally helpful and rarely harmful". (whether this comes under that may be a different matter!)
For the OLE2 and Zip cases, we do provide more accurate detectors, which will only run for files with the right initial mime magic, so people who care about greater accuracy (at the expense of a little more processing time) can make use of that if they choose
For your specific case, you only need to check the first 4 bytes to know if a file has the Zip or OLE2 mime magic. It may be best to have code that tries the first few bytes from your truncated stream, if it matches then it can pass the whole file to the appropriate container detector, and if not it can pass the first few kb to the regular mimetypes code. That's likely to be less brittle, as well as easier to follow. It should also cope well for adding other container detectors (eg Ogg) later.
(Most people can simply pass in the whole stream to DefaultDetector and have something like this done for them, it's only special for you because you want to detect most files off of the initial few kb, with the whole file for certain types)