I am not able to Drag n Drop documents in Master Page Gallery in SharePoint 365 Trial.
How to enable it?
Custom scripting capabilities are disabled by default in the tenant. Because of that, you are not able to drag-drop in the master page gallery. It also affect other features such as not able to use Script/Content Editor webparts, custom master pages , page layouts etc. Have added link at the bottom of the answer. Open the section,
Features affected when scripting is disabled in the link.
Go to the SharePoint admin centre.
Under Custom Script choose below options and save the settings:
After that, open the SharePoint online powershell and run the below commands. You need to be SharePoint admin for the commands to work:
Enter URL of admin site
Then username and password and after that run the below command with your site collection specified.
Set-SPOSite -Identity https://tenantname.sharepoint.com -DenyAddAndCustomizePages $false
After that, wait for like 2-3 mins, close and open designer again. It should work.
Reference - Turn scripting capabilities on or off
Download link - SharePoint online powershell
From my experience, that's in the design of "SharePoint 2016" model. Therefore it will be either a lot of clicking - or - you can also map your Master Page Gallery as a network drive, or open the Gallery in the Explorer View to faster handle the file management.
Before you can upload or create files in a library, your admin must give you permission to contribute to the library. Also, some file types in the library are blocked for security reasons. See Types of files that cannot be added to a list or library for more info.