The Power Platform admin center features a section to define valid file type extensions or mimetypes. This is used to validate files that are uploaded to a Dataverse file column or attachment table.

HOWEVER, this validation is not used at all when using the SharePoint integration in Power Pages. Please modify the integration to either use the existing file type validation, or provide an interface to define valid file types when adding the integration to the form itself. In either case, this validation needs to be performed on the server side so that it cannot be bypassed in the UI.