On a content of content type File, when you change an already uploaded file from one with a mimetype with the format "a/file" to another with mimetype "b/anotherfile", the downloaded file after updating the content will save the file with the original file mimetype's affected extension.
Example:

• Create File content with a file image.png (thus having mimetype "image/png"),
• publish it,
• download it from its showing link: it will be saved as "<image_hash>.png

Edit the content and replace the file with video.avi (having mimetype "video/avi"),
republish it,
download it: it will be saved as "<image_hash>.png", still, instead of as an avi extension file.

Update: Mimetype doesn't actually matter - if you replace the file with another one of the same mimetype, it still downloads with the previous filename.