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**Summary** Do not assume MSG format when an OLE "container" file cannot be differentiated into DOC, PPT, XLS, or MSG. Fall back to extention-based identification in that case. **Additional Context** DOC, MSG, PPT, and XLS are all OLE files. An OLE file is, very roughly, a Microsoft-proprietary Zip format which "contains" a filesystem of discrete files and directories. An OLE "container" is easily identified by inspecting the first 8 bytes of the file, so all we need to do is differentiate between the four subtypes we can process. The `filetype` module does a good job of this but it not perfect and does not identify MSG files. Previously we assumed MSG format when none of DOC, PPT, or XLS was detected, but we discovered that `filetype` is not completely reliable at detecting these types. Change the behavior to remove the assumption of MSG format. `_OleFileDifferentiator` returns `None` in this case and filetype detection falls back to use filename-extension. Note a file with no filename and no metadata_filename or an incorrect extension will not be correctly identified in this case, however we're assuming for now that will be rare in practice.