Microsoft Teams is a really easy way to share files with others. However, the modern concept with Microsoft Teams is that once you are part of the Team then you have the same rights as everyone else. This generally means that all Team members have the ability to read, write, modify and potentially delete files. This is common across all channels in the Team.
One thing that you really don’t want to do is go into the SharePoint back end of the Teams files and modify the default permissions. If you do, you’ll cause a whole lot of problems. We are expecting private channels in Teams very soon but here’s an easy way to overcome the default common sharing options in Teams by creating a separate area with unique permissions and linking that back into the Team.

Firstly navigate to your Team.

Select the Files tab to the right of Conversations to see all the files for that channel as shown above. These are common files that all Team members have the same rights to.
Select the Open in SharePoint option as shown above.

This will take you to the location of those channel files in SharePoint as shown above. This location is typically a subfolder with the name of the channel (here General), in a Document Library called Documents.
You will need appropriate permissions to complete the process from here. So you will need to be an admin of the Team or a SharePoint Site owner.

In the top right of the screen select the COG then Add an app from the menu that appears as shown.

Typically, you’ll select to a new Document Library and give it a name.

In this case, a new Document Library called Final Presentations has been created as shown.

Once you are at this new location, select the COG again in the top right and this time select Library settings as shown.

Select the second option from the second column at the top of the page called Permissions for this document library.

Now it is just good ol’ SharePoint permissions configuration.
Typically, you firstly select Stop Inheriting Permissions.

In this case, Sales members will be changed from Edit to Read permissions by selecting that group and then the Edit User Permissions button. However, you can configure whatever permissions suit your needs.

Make sure you select OK after you have made you changes.

Once you have completed the require permissions, you need to return to the Team and link this new location there.

Inside the Team, select the channel in which you wish this new location to be linked and select the + icon on the right as shown.

From the dialog that appears, select Document Library as shown.

You can either navigate or input a direct link here. In this case the destination site, Sales, is selected.

You should then see the new location you created (here Final Presentations). Select this and then the Next button.

Give the new tab a name, which can be different from the location if you wish, and press Save.

You should now see the location you created and any files in there as shown above. These items have permissions governed by those set previously in SharePoint but now they are also displayed and accessible in Teams. The great thing is you can link this new location in multiple places and you can link from locations not even in the current Team. As long as users have permissions, they can see and interact with those files based on those permissions.
Hopefully, that is an easy way to create locations for file with unique permissions but still have them accessible for users via Teams.