fix mime type from watermark #1540

Job	Run time
Build Matomo NGINX / build	18s
Build Matomo PHP / build	20s
Build Keycloak / build	27s
Build Databox Indexer / build	17s
Build nginx-cache-purge / build	21s
Build nginx-fpm-base / build	15s
Build nodejs-base / build	15s
Build php-fpm-base / build	25s
Build Report API / build	21s
Build Databox Client / build	18s
Build Expose Client / build	29s
Build Dashboard / build	1m 53s
Build Uploader Client / build	27s
Build Configurator / build	26s
Build Notify API / build	1m 19s
Build Databox API / build	50s
Build Expose API / build	1m 24s
Build Uploader API / build	1m 25s
Setup stack	0s
	11m 30s

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