One or more mimetype migrations are available. Occasionally new mimetypes are added to better handle certain file types. Migrating the mimetypes take a long time on larger instances so this is not done automatically during upgrades. Use the command occ maintenance:repair --include-expensive to perform the migrations.
Please describe IN DETAIL what you’ve tried to do, in which order, etc., etc… Due to the fact that you’ve completely ignored the support template and didn’t provide valuable information about your environment it is nearly impossible to give you any advice how to solve the issue.