User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0

Steps to reproduce:

Send yourself a message with a text file attachment. In the attachment list, right-click the attachment, select "Detach". Save the attachment some where.
Open the attachment area again.

Actual results:

The attachment shows a never-ending spinner.

Expected results:

The spinner shouldn't show. It's only for external attachments which are being fetched.

Alice, can you please find the regression.

Works fine in TB 95 beta, so it's a very recent regression. Instead of the spinner, you should see the icon belonging to the attachment type, like if it weren't detached.

I cannot reproduce the issue in ether Tb75.0b3 and Daily96.0a1(20211115103144) Windows10.

Open the attachment area again.

Right click on the attachment and choose Open. The attachment will be opened in my text editor as expected.

Aha

Open the attachment area again.

You mean Show the attachment pane. i.e, Click on > mark to expand the attachment list.

The icon of a attachment is spinning forever.

Regression window:
https://hg.mozilla.org/comm-central/pushloghtml?fromchange=91332386d3ddc8d5e28c2fc729ad5d7ca821c8e2&tochange=d5a2efeb62c6a21b4e2f678be7fac858bdb3624e
https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=300fc6bd088e8568d61395252c1b0bcb6fa9c4b5&tochange=300fc6bd088e8568d61395252c1b0bcb6fa9c4b5

Must be from bug 1667652 - https://hg.mozilla.org/comm-central/rev/73a0f03a037dcb0afcf685c20cde18492d2b1eb6

I will fix it.

Backed out bug 1667652 so this is now resolved.

I think you guys found a second bug in your STR. That should actually be the detached icon, not the default/standard icon, after the attachment has been detached. At least we have this in the code:
https://searchfox.org/comm-central/rev/fc1f4d8339c84577090fda2e716059074e11430b/mail/base/content/widgets/mailWidgets.js#1603-1604

This used to work in TB78 (see screenshot), but it no longer works like this in TB91.

Have we changed that deliberately, or should I file that as a new bug?

Detached and deleted are two different things. Only a deleted attachment will have the word "Deleted:" prepended.
https://searchfox.org/comm-central/rev/fc1f4d8339c84577090fda2e716059074e11430b/mailnews/mime/src/mimemult.cpp#245
There's a bug to localise that, bug 323109. As far as we can tell, everything works in 91.

Ah, correct, thanks for the explanation.