If we carefully look at the names of our files, we will find that there is a three letter or four letter suffix followed by a dot after the name of the file. This suffix is called the filename extension. In fact, this is the encoding or file format by which the computer programme recognises the file to open it, if it is associated with that programme. Some common file extension names are .doc or .dox for word documents, .xls for Microsoft Excel files, .png, .bmp, .jpeg etc. for image files, .exe for executable files, .bat for batch files, .txt for text files and so on. The file systems like he FAT file systems or NFTS limit the length of the file extension. This is the reason we find that the most of our file extension names are limited to three letters and occasionally four letters. The separator dot is accepted as a legal character by the Unix filesystems. The filenames are a type of metadata giving information about the way how the data is stored in the file. The file extensions in MS DOS and MS Windows with extensions like exe, com or bat indicate that the file names with these extensions are the programme executable files.
In case, we change the file extension, then the file will not open since the system cannot recognise the type of the file format and the programme with which the file is associated will not be able to open it. We will usually be asked the information about the programme with which the files is associated to open the file. So, we should not change the file extension name of the file, which sometimes we do accidentally while changing the name of our files.
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