File Name Structure

On most operating systems, including Windows, file names are divided into two parts, with each part divided by a period. The first part is the name of the file, and is usually unique and descriptive for the contents of the file.

File Extension

The second part, usually three characters but sometimes more, is the file extension. This identifies a type of file. For example, TXT identifies a plain text file and DOC identifies a Word document.

The TMP File Extension

The TMP file extension is usually used to identify a file that is temporary or contains information that will be in the file only temporarily.

File Association

To make computer use easier, operating systems associate certain file extensions with programs that can access them. For example, clicking on a TXT file will open Notepad. Clicking on a DOC file will open MS Word.

Setting File Associations

TMP files do not have a file association because they are used by many programs and are usually not meant for interaction with an end user. But if you as a user are certain of one program that you would like to associate with a file, you can set the file association using the Explorer.

Source:

How to Change or Select Which Program Starts