Reviewing Large Message Attachments

I am nearing the storage limit on my phone so I’m trying to make space. It looks like a lot of my storage is being used by messages so I was reviewing large message attachments. I’m doing so, I’m noticing several files that instead of having a preview image for their thumbnail, they have a blue clip thumbnail.

• What does the blue clip icon mean?
• I assume the icon means the files are “missing” and/or can’t be saved for messages. If so, then are they even taking up space? It appears as if they are since they’re showing up under large attachments. But that also seems strange that they would show up there if they’re “unavailable.”

Ideally I’d like to save all attachments that were not sent by myself.