`Lectures`: Improve lecture attachment validation #12264

Job	Run time
get-labels	4s
check-build-status	5s
filter-matrix (artemis-test1.artemis.cit.tum.de, artemis-test1, https://artemis-test1.artemis.cit...	1s
filter-matrix (artemis-test2.artemis.cit.tum.de, artemis-test2, https://artemis-test2.artemis.cit...	1s
filter-matrix (artemis-test3.artemis.cit.tum.de, artemis-test3, https://artemis-test3.artemis.cit...	1s
filter-matrix (artemis-test4.artemis.cit.tum.de, artemis-test4, https://artemis-test4.artemis.cit...	1s
filter-matrix (artemis-test5.artemis.cit.tum.de, artemis-test5, https://artemis-test5.artemis.cit...	1s
filter-matrix (artemis-test6.artemis.cit.tum.de, artemis-test6, https://artemis-test6.artemis.cit...	1s
filter-matrix (artemis-test9.artemis.cit.tum.de, artemis-test9, https://artemis-test9.artemis.cit...	1s
process-matrix	1s
deploy (artemis-test4.artemis.cit.tum.de, artemis-test4, https://artemis-test4.artemis.cit.tum.de...	1m 18s
add-error-label	0s
	1m 35s

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