Instead of using FileManager widget you can use File Dropper widget ( https://marketplace.mendix.com/link/component/111497 )
We can upload multiple no of file at a time by using this. To View this file you can use Universal (https://marketplace.mendix.com/link/component/116935 ) file viewer widget.
Here you need to configure the Entity (Attachment) which is generalized from System.FileDocument and one Association (MyWorkflow.Attachment_Comment/MyWorkflow.Comment) .
Note: FileDropper must be placed inside a Dataview only.
Highlighted in yellow is widget you can click that and select a files (txt,pdf,jpg), that will be previewed as bellow the widget.
Above are retriving the list of uploaded Images. I Hope this info will be helpful for you. Thanks.
Please refer this link
Upload a File in Your App Using ATS - Quality Add-ons Guide | Mendix Documentation