I just found a solution to fix EXL-300873 issue wherein excel file with .xlsx can’t be accessed/read in BODS 4.0 version. I think in DS 4.2 this is already taken care. The reason in my opinion is the access adapter that we use.
Temporary solution is to reinstall MS Database Access Engine and reboot the server. Sometime only server reboot works. But this is not advisable to reboot DS server now and then.
Permanent solution would be to install complete office suite 2013(or latest if you expect your source files created on 2013 or higher version of Excel). If you have space constraint on server, just install Excel, Access, shared and other office component using CUSTOM install option. Indeed a server reboot is required to make office components work. Try and share your results.
p.s. EXL-300873(OLE or COM processing error. Please make sure Microsoft Access DataBase Engine is properly installed) is a message that can be misleading sometime. The above will work for <External table is not in the expected format.>