Normally when you try and open a file for which the extension is not recognised, Windows will inform you that there is no associated program and may ask you to choose one from a list.
If you are in any doubt as to whether the attachment is safe to open, don't open it. Further information here.
The most common reason for this is that the attachment has an extension of a type not recognised by software on the device - for example if the attachment is a Word document, which has the extension .doc then the PC will try to open it with MS Word, likewise .xls files will be opened in MS Excel. These are commonly known as File Associations.
Occasionally you may receive an attachment that has no extension, e.g. instead of my_essay.doc it may only be called my_essay - if this is the case then you can try to determine the extension, preferably by contacting the sender of the original message, but you may have luck selecting a program from the 'Unknown File Type' window mentioned above.
In some instances Windows may recognise the file but have trouble for other reasons, particularly when a file is corrupt. A good indication of this is when an application, such as Word, tries to load but then hangs.