Windows generates short file names from long file names in the following manner:

• Windows deletes any invalid characters and spaces from the file name. Invalid characters include:
  . " / \ [ ] : ; = ,
• Because short file names can contain only one period (.), Windows removes additional periods from the file name if valid, non-space characters follow the final period in the file name. For example, Windows generates the short file name
  Thisis~1.txt

  from the long file name

  This is a really long filename.123.456.789.txt

  Otherwise, Windows ignores the final period and uses the next to the last period. For example, Windows generates the short file name

  Thisis~1.789

  from the long file name

  This is a really long filename.123.456.789.
• Windows truncates the file name, if necessary, to six characters and appends a tilde (~) and a digit. For example, each unique file name created ends with "~1." Duplicate file names end with "~2," "~3," and so on.
• Windows truncates the file name extension to three characters or less.
• Windows translates all characters in the file name and extension to uppercase.

Note that if a folder or file name contains a space, but less than eight characters, Windows still creates a short file name. This behavior may cause problems if you attempt to access such a file or folder over a network. To work around this situation, substitute a valid character, such as an underscore (_), for the space. If you do so, Windows does not create a different short file name

For example, "Afile~1.doc" is generated from "A file.doc" because the long file name contains a space. 
No short file name is generated from "A_file.doc" because the file name contains less than eight characters and does not contain a space. 
The short file name "Alongf~1.txt" is generated from the long file name "A long filename.txt" because the long file name contains more than eight characters.