Usually, if there are huge attachments (of size more than 25MB) on any knowledge records, they get opened up in a new tab and a preview is shown.

This generally happens if you have a video (.mp4 or .mov) type attachment on any KB articles. These huge attachments will be shown as a preview in a new tab when the user clicks on them. There is a possibility that the attachments (.mp4 or .mov) will take a lot of time to load while showing the preview. Also, user cannot perform/carry out other transactions while the video preview is loaded.

This behavior of showing video preview can be avoided. There are two system properties which control the way attachments are previewed/downloaded in the ServiceNow platform:

1) 'glide.ui.attachment.force_download_all_mime_types' and set the value to true.
This will cause the attachments to download when the user clicks on them (preview will be disabled)
[OR]
2) 'glide.ui.attachment.download_mime_types' to force download only mp4/mov video type attachments instead of showing a preview
Add "video/mp4" [or] "video/mov" according to the video types you want the user to force download.

In either of the above methods, when the user clicks on a video attachment on the KB article in the service portal, the attachment will get downloaded to the user's local machine.
This way we can avoid the preview video loading time issue. (Also, please be noted that until the file attachment gets downloaded completely on users local machine, user cannot navigate through other ServiceNow pages, which is a platform behavior)