EXPIRED –This file is no longer available for download. Ask the sender to send it again.

You sent a large file using iCloud. That file expires after 30 days. You'll need to send the video to yourself again from the original source.

Handel wrote:

What a dumb system.

30 days isn't long enough and there should be some back-up.

An attachment that exceeds the limits of your email provider is sent via MailDrop, a FREE apple service that uploads the attachment to Apple’s servers so it can be downloaded by the recipient, as described here→Send large attachments in Mail on Mac - Apple Support

Quoting from this article:

You can use Mail Drop to send files that exceed the maximum size allowed by your email account provider. Mail Drop uploads large attachments to iCloud, where they’re encrypted and stored for up to 30 days.

Which is actually generous, because Apple is providing this as a free service.

MailDrop serves as a conduit to deliver attachments that otherwise exceed the mail servers limits. It is ephemeral and allows the recipient 30 days to download the files for posterity.

Handel wrote:

What a dumb system.

30 days isn't long enough and there should be some back-up.

I sent a collection of photos last year to a friend who is now dying of pancreatic cancer, but can't remember which ones. Few of us have perfect memories regarding the 'original source'. Important documents shouldn't just 'expire'.

What's the problem? Shouldn't my emails, with attachments, be kept for as long as I want them, in my (big, largely unused) iCloud account?

Important documents don't "expire". What expires is the file upload in your email. Most email accounts have limits on the size of files and how long they store things on their servers.

You need to be responsible for storing and maintaining your documents. I recommend a couple of external hard drives.

What a dumb system.

30 days isn't long enough and there should be some back-up.

I sent a collection of photos last year to a friend who is now dying of pancreatic cancer, but can't remember which ones. Few of us have perfect memories regarding the 'original source'. Important documents shouldn't just 'expire'.

What's the problem? Shouldn't my emails, with attachments, be kept for as long as I want them, in my (big, largely unused) iCloud account?

Are you saying we have to back up any email with an attachment we send, to a ‘couple of external drives’? Why doesn’t the email with an attachment get stored automatically in my (largely unused) ICloud file?

No, I'm saying you need to back up your attachments and documents. And, if they're really important to you, in addition to syncing to iCloud, you'll have them on a device that you have complete control over. And you should always have a second back up. If you don't do that, hard though it is to hear, you have no one to blame but yourself. I think all of us have lost data at some point because we didn't take backing up seriously. And most of us learn from those painful experiences.

You actually are saying I have to back up my attachments.

I don’t understand why attachments in emails, at danger of being destroyed by the Mac system aren’t automatically backed up into my iCloud account.

I already have 2 virtuous external hard drives used for backups but, not having an IT personality, I hadn’t realised collections of photos in emails were at hazard.

Handel wrote:

You actually are saying I have to back up my attachments.

Yes, that is exactly what I said. Glad we're in agreement on that.

I don’t understand why attachments in emails, at danger of being destroyed by the Mac system aren’t automatically backed up into my iCloud account.

iCloud is not a backup service for Macs. It's a syncing service. Forget that at your (data's) peril.

I already have 2 virtuous external hard drives used for backups but, not having an IT personality, I hadn’t realised collections of photos in emails were at hazard.

I don't know what a "virtuous" hard drive or an "IT personality" is. I'm not a banker but I still know how to put money in a savings account.

what was video's name. You might be able to search for on the net.