Hello,
removing mimetypes from these files does not only "hide" it in Share, but makes them fully unknown to Alfresco. This can - in some cases - lead to some files not being handled correctly in terms of thumbnail / preview rendering. If you want to hide some mimtypes in the selection lists of Share forms, you might want to use the selectone.ftl control template instead of the default mimetype.ftl, and explicitly configure the mimetypes that you want to be available as options.
Regards
Axel