Extension (file name extension) - a sequence of characters in the file name that allows the user and the software to identify the type of data contained in the file.
The most common location of the extension - the final characters in the file name, separated from the main name by the last dot.
The file named picture.jpg has the extension jpg and will be identified by the system as an image file. The file named picture.jpg.n has the extension n and will not be identified by the system due to the lack of viewers comparable to this extension.
The extension indicates a format in a broad sense, but is not a file format.
So how does the extension differ from the format? An extension often does not provide complete information about the format and, accordingly, about the file itself, for example, about the codecs used, encodings, and so on.
It is important that the file extension, by and large, does not affect its integrity.
The file named picture.jpg.n, if the last two characters were simply added manually, you can open it in most image viewers, if you independently initiate playback or compare the viewer with extension *.n
Format (file format) - definition of the type of structuring and specification of the file content, which sets the rules for its writing and reading.
Most often, the format is indicated by the extension in the file name, but it should be understood that a file with an incorrect or incorrect extension, as well as a file without an extension, still retains its structure, that is, the format. Consequently, such a file can be played with the appropriate program. This can be taken as the main feature by which the format differs from the extension.
It follows from this that data format is a concept that is much more complex and precise, which is of more systemic importance than the file extension.
If the image file picture.jpg is renamed to picture.mp3, it will not be able to play it as a music file, of course, but you can open it as an image.
Resolution (screen, image) - generally indicates the number of dots (pixels) horizontally and vertically, which characterize the technical part of the screen or the size of the image.
The concept has nothing to do with Expansion, except for the consonance of names in some languages. For example, in Russian: Razresheniye (Resolution) and Rasshireniye (Extension).
On the screen with a resolution of 1600x900 we open the image in JPEG format with a resolution of 800 x 600, having the extension *.jpg