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Besides the .txt extension issue (Microsoft Windows may hide extensions of known file types), you want to make sure that the text editor doesn't add a UTF-8 BOM (Byte Order Mark) or a line break at the end of the file (either explicit or implicit, the latter on Unix-like operating systems). The mimetype file has to have the size of exactly 20 bytes.
Calibre doesn't create EPUB3s, it might be usable to edit EPUB3 and keep the result valid, if some buttons aren't pushed.
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