I upgraded my laptop to Mint 20.1
If I download a file to my desktop I get a proper icon of the kind that reflects the contents.

If I move or copy it to a nemo window, I just get a generic icon that reflects the file type.

This applies to PNG, PDF, JPEG and others.

My laptop has a lot of crud on it, but even so...

A little more info about your setup:

• desktop environment
• user customisations
• what was the situation before the upgrade

Did you check nemo's settings?
What is managing the desktop? Nemo, too?

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I'm using Mint 20.1 , Cinnamon with Nemo 4.8.4, no significant customisations.
However, thanks to your comment:-
led me to check a toolbar setting on Nemo.

There is an optional icon to toggle showing thumbnails - not sure why its a good idea - confused me anyway.
The desktop [also controlled by nemo] automatically has it set on.

So it's fixed.

Attached Images

 

Because some people don't care for them, and because they can be a drag on resources, esp. with large images and PDFs and who knows what else.
Configurability is a Good Thing.
So toggling that solved it. Good.

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I held off updating to Mint 21 until yesterday. My (quite similar?) issue is that, while I eventually figured out the toggle for showing thumbnails, Nemo no longer generates any icons for mp4 files. I looked in all the usual places, checked size limitations, etc. Based on several comments on line I also installed xplayer (which I never use) and ffmpegthumbs (ffmpegthumbnailer was already installed). Rebooted, etc. but no joy.

Any ideas, or would it be better to just do a fresh install (my usual approach), since all my files are on other drives/partitions. I did notice some quirks in the update process (e.g. my custom 40_ grub additions were all doubled up, LibreOffice settings were wiped out, one of my input methods disappeared, etc.), though these were all easily fixed.

I'll keep poking around for a bit before going through a new install, but does anyone have any suggestions as to what I (or the update scripts) might have missed?

Thanks ...

Seems to me the Linux Mint developers do not do any (or enough) regression testing. They make a change and never know they broke something (or changed something)that use to workright after install. This is a perfect example that has been broken or the user forced to do some undocumented thing to get it back to work. (I'm a software developer for over 20 yers)