The first time you try to export a file in GIMP, you'll probably get a message that says something like this: "The given filename does not have any known file extension. Please enter a known file extension or select a file format from the file format list." This may look a bit surprising since normally you expect saving a file to just work, but in GIMP it doesn't work like that for some reason.
Why it Happens
GIMP uses the file extension you type in the dialog box to figure out which format it should use when saving a file. This means if you type a filename without an extension, GIMP won't know how to save a file, and instead of showing a dialog to let you pick an extension or a format, it shows a message that says you didn't type an extension it knows of.
How to Fix
There are two ways to fix this, as cryptically written in the message box.
The first way is to type an extension for the filename into the export dialog. For example, if you want to save a file called photo in the JPEG format, you need to type photo.jpg, because .jpg is the standard extension of the JPEG image format.
If you're a Windows user you may be unfamiliar with file extensions because Windows hides them by default.
For reference, common image file extensions are .jpg, .png, .gif, .webp, .bmp, and .tiff.
A second way is to click on "Select File Type (By Extension)" at the bottom of the export dialog. This widget will show you a list of GIMP's supported formats. Selecting an option on this list automatically changes the filename to match that option's extension. For example, if you type photo as filename and select JPEG image as type, it will automatically change the filename to photo.jpg.
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