If you have added a property to an existing model with the type of "file" and added the property as a field in a form on a page but when you go to upload a file, you get this error:
You need to change your field's configuration.
On the Admin home page, select Pages and open the page with your form on it.
Select the settings cog and on the Content page select CONTINUE.
In the Design form section, select the field with the file-type property and select the edit pencil.
In the Edit field dialog box, scroll to the bottom and from the Types of files list, change the setting from Images to Documents or All. Select UPDATE.
On the Content page, select UPDATE.