What a MIME type is
A MIME type, also called a media type or content type, is a short label in the form type/subtype that tells browsers, servers and email clients what kind of data they are handling. Common image types are image/jpeg, image/png, image/webp, image/gif, image/avif, image/tiff, image/bmp, image/x-icon and image/svg+xml. HEIC photos use image/heic or image/heif.
Web servers send it in the Content-Type header, and upload forms often check it to decide whether to accept a file.
Why the file extension can lie
Your operating system and browser usually guess a file's type from its extension. Rename photo.png to photo.jpg and they will happily call it image/jpeg, even though nothing inside has changed. Images saved from websites are a frequent source of confusion: a WebP file is sometimes downloaded with a .jpg name.
Stricter services look at the file's actual bytes. When the real type doesn't match the name or isn't on their allowed list, the upload fails with a vague invalid file type message. This checker decodes the file on the server and reports the MIME type of the format it really is, so you can see the mismatch for yourself.
How to fix a mismatch
If the service accepts the real format, renaming the file with the correct extension is enough. If it requires a specific type, for example JPEG only, convert the file: WebP to JPG or PNG to JPG produce a genuine image/jpeg file, not just a new name.
For developers, the lesson is the same in reverse: set Content-Type from the decoded content rather than the uploaded file name, and validate uploads by reading them.