If you found that you can't open eml files in attachment mode from your outlook express and show that is unknown files type, that is because your windows missing recognition of eml files type so you might need to do the following steps: -

RESOLUTION

To resolve this behavior, either run the Msimn executable file with the /reg option to force Outlook Express to re-register its file types, or re-register the .eml file type in My Computer.

To run the Msimn executable file with the /reg option:

1.	Quit Outlook Express.
2.	Click Start, and then click Run.
3.	In the Open box, type msimn /reg, and then click OK.

To re-register the .eml file type in My Computer:

1.	On the desktop, double-click My Computer.
2.	On the Tools menu, click Folder Options.
3.	Click the File Types tab.
4.	Under Registered file types, click EML Outlook Express Mail Message, and then click Advanced.
5.	Click Open, and then click Edit.
6.	Type the following single line in the Application used to perform action box: "C:\Program Files\Outlook Express\MSIMN.EXE" /eml:%1
7.	Click OK, click OK again, and then click Close.

This solution has been tested and confirm it's work. TQ.