Fix when att mimetype has no guessed extension (#386) #1583

Job	Run time
test / test	1m 7s
version	16s
release-notes	3s
pypi	53s
pyinstaller (macos-14) / pyinstaller	1m 18s
pyinstaller (macos-12) / pyinstaller	2m 15s
pyinstaller (windows-latest) / pyinstaller	3m 17s
package-prep	14s
package-metadata	7s
flatpak	48s
package-macos (macos-12)	28s
package-windows	1m 9s
package-windows-msix	45s
package-macos (macos-14)	29s
deploy-msstore	0s
deploy-github	11s
pr-comment	0s
	13m 20s

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