Scan files for viruses on upload? Doesn't that depend on the scanner package having a profile to look for? Which means that a new virus gets through until someone circulates a new profile (and other people fetch it to update their scanners)?

If a file is supposed to contain an image (but also contains a virus), it actually makes a lot more sense to apply some global filter to the "pixel" data such that image quality would be generally preserved, but anything like machine code would be obliterated. It's a pity that the same does not hold for email attachments that are supposed to be MS-Word "doc" files, etc...