This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Technically you can, but as William said Windows will not see that the file is not a text file or Word 2007 file. Therefore, double clicking the file will have Windows open the file with Notepad (or any other editor that is configured to open .txt files) or Word 2007. These programs can't properly display these files.
.ser is often used for files containing serialized objects, both because it makes it a bit clearer what the file contains and because .ser is not an extension used by popular applications. If you want you can use .vishal or any other thing you like. I usually use no extension at all.