Microsofts Outlook e-mail program sometimes sends e-mail attachments as "Transport Neutral Encapsulation Format" (TNEF).
This attachment is usually named "WINMAIL.DAT".
If you are missing attachments you can use this free tool to extract the attachments from the winmail.dat file.
How to open a winmail.dat file
Select Winmail.dat Opener from the context menu of the winmail.dat attachment.
Start Winmail.dat Opener and use the built-in file dialog to select a previously saved winmail.dat file.
Once you are at the contained files list, you can long click a file to open it or select files and click the "save" icon from the action bar to save all selected files to the sdcard.
All files will be saved to a directory called "winmaildatopener". After successful export of a file to this directoy an open file dialog will enable you to start further actions, like open, share on the exported files.
Not all Android email apps allow a fine enough attachment-to-application mapping.
Therefore with some email clients you can not start the extraction process via the context dialog in the email client.
Instead you have to first save the file to the storage and then open it with the built in file dialog of the "Winmail.dat Opener" app.
They other solution would be to make a very broad mapping, but then you would have this app in every chooser dialog which would be even more annoying.