fix: check ole storage content to differentiate filetypes

[MthwRobinson]

Summary

Updates the file detection logic for OLE files to check the storage content of the file to more reliable differentiate between DOC, PPT, XLS and MSG files. This corrects a bug that caused file type detection to be incorrect in cases where the filetype library guessed and incorrect MIME type, such as 'application/vnd.ms-excel' for a .msg file.

As part of this work, the "msg" extra was removed because the python-oxmsg package is now a base dependency.

Testing

Using a test .msg file that returns 'application/vnd.ms-excel' from filetype.guess_mime.

from unstructured.file_utils.filetype import detect_filetype

filename = "test-file.msg"
detect_filetype(filename=filename) # result should be FileType.MSG

[MthwRobinson]

Test was removed because the extension is no longer required to detect MSG files.

[scanny]

Sweet! LGTM :)