Fix when att mimetype has no guessed extension (#386) #1582

Job	Run time
test / test	1m 15s
version	16s
pypi	59s
release-notes	3s
pyinstaller (macos-14) / pyinstaller	1m 11s
pyinstaller (macos-12) / pyinstaller	2m 33s
pyinstaller (windows-latest) / pyinstaller	4m 47s
package-prep	18s
package-metadata	4s
flatpak	41s
package-macos (macos-12)	31s
package-windows-msix	48s
package-macos (macos-14)	27s
package-windows	1m 7s
deploy-msstore	0s
deploy-github	12s
pr-comment	0s
	15m 12s

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