1. A computer-implemented method for protecting a computing system from untrusted computer files, at least a portion of the method being performed by a computing device comprising at least one processor, the method comprising: identifying a point in time before which at least one security scan of file a plurality of computer files has been performed by a security system; identifying, within the plurality of computer files, by the computing device, a computer file that: has a timestamp indicating that the computer file was created or last modified before the point in time; and is currently not assigned a security classification; assigning, by the computing device, a security classification to the computer file that indicates the computer file is not trusted; and performing, by the security system, a security action to protect the computing system from the computer file based on the assigned security classification of the computer file.