1. A device for identifying types of document files comprising: an extraction unit that extracts document feature information indicating distinguishing information for a document included in a document file; an evaluation unit that verifies the validity of the document feature information extracted by the extraction unit; and a document type identification unit that identifies a type of the document file corresponding to the document feature information extracted by the extraction unit if the document feature information is valid, wherein the document file is one of a plurality of types, and the document feature information is different for each type, wherein the device further comprises an extraction method storage unit that stores extraction method information describing a plurality of methods used to extract the document feature information from the document file for the respective types of the document files, and wherein the extraction unit extracts the document feature information from the document file according to the extraction method information stored in the extraction method storage unit.