Thank you for getting in touch. I took a look through some of the apps on the Shopify App Store that allows for customers to upload content but, as you mentioned, they are primarily designed for customers to upload files on product pages. I did come across Upload-Lift Image Upload which claims to allow customers to "upload files directly on Product, Contact, Cart and Order Status pages", so it may be possible to allow for an upload field to appear on other pages using this app. I would recommend contacting the app developers at firstname.lastname@example.org to ask them if what you want to achieve is possible with their app and they should be able to confirm this for you.
If not, and if you cannot find a suitable app for your needs, you'd need to hire a Shopify Expert to build this functionality for you.
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