"Next" button disabled after 20th file in Media Viewer

[meelisl]

Describe the bug

When an item contains more than 20 files/images, the Media Viewer stops at the 20th image — the "Next" button becomes disabled and does not allow browsing further.

To Reproduce

Create an item.

Add more than 20 images/files.

Open the item in Media Viewer.

Try to navigate beyond the 20th image using the "Next" button.

Expected behavior

The Media Viewer should allow browsing all added files using the "Previous" and "Next" buttons — not just the first 20.

Test example

A test item demonstrating the issue is available on the DSpace Demo site:
https://demo.dspace.org/entities/publication/315f9c75-0a10-42de-bbcc-62384f5292e9

[tdonohue]

Verified on demo site (currently running 9.x) and reported on 8.x via this email thread: https://groups.google.com/g/dspace-community/c/03ukDFCcYN4

(This likely also impacts 7.x, but unverified at this time.)