History:
File extensions originally started in the days of DOS. A typical file name was given in the 8.3 file format where 8 letters were the file name followed by a dot and three letter for the file extension. Historically that was as long as a file name could be.
Eventually Microsoft did introduced long filenames along with an extended version of the FAT file system.
Multipurpose Internet Mail Extensions (MIME) is a specification for to format non-ASCII messages so that they can be sent over the Internet. In addition to e-mail applications, Web browsers also support various MIME types. This enables the browser to display or output files that are not in HTML format.
Link to a gif file chili.gif
This has a .gif extension and a mime type/subtype of image/gif
If you click on the linkabove, the browser will open a window and show you the picture.
If you right-click on the file
- save it to your computer
- double click on the file (open with the default .gif application)
- you will also see the picture.
Link to a gif file chili.txt
Here the file extension has been changed to .txt but still has a mime type of image/gif (same file but the extension has been changed). If you click on the linkabove, the browser will still open a window and still show you the picture.
If you right-click on the file
-
save the file to your computer
- double click on the file (open with the default .txt application)
- you will see a bunch of garbled text. The first few letters tell us that it is in the GIF89 file format.
Should you run across a file that does not have an extension - it may not work correctly without the correct extension. If you add the .txt extension and open the the file there might be some text at the very beginning of the file that will shed some light on what application (and therefore extension) the file was made with.
If you see |
It is probably |
| Standard Jet DB |
Access file |
| %PDF |
PDF file |
| PK |
ZIP file |
| MZ |
executible file |
| BM |
BMP bit mapped image |
 As a last resort, if you have a file in question you might want to try an online tool called TrID File Identifier. I created a Word file and deleted the file extension. The file lost the Word ICON associated with it. I ran it through the TrID File Identifier process and it did recognize it as a Word File and suggested that it should have a .doc file extension. Although there are ways to determine a file type it is still recommended that you not open files that you have questions about unless you are certain of the file content and the source.
Often email servers will strip files of certain types for security reasons. On my email server there are reasons that I must send Access files to other people. These files get stripped because of the security settings on the email server. I can change the file extension to .txt and send the file with a message to the intended recipient for them to change it back to a .mdb file extension that associates the file back to Access. This is the only way I can send an access file through my email server.
The way file extensions and mime types work together can be a little complicated at times. This is NOT a thourough discussion of how they work but an attempt to at least make you aware of how they work.
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