.thumbnails is Android Thumbnails Folder
Key Features: .THUMBNAILS File Extension
What is an .THUMBNAILS file?
A folder with a .THUMBNAILS extension is a hidden folder stored in the sdcard/DCIM directory on select Android devices. It contains one or more .thumbdata files that store properties about thumbnail images indexed by the Gallery app to load images quicker. THUMBNAILS folders typically store .THUMBDATA3-1967290299 and .THUMBDATA3-1763508120 files.
You can navigate to the THUMBNAILS folder on an Android device using a file manager. However, you need to verify that your system displays system and hidden files in the settings.
The more images you have in the Gallery app, the larger the THUMBDATA3-1967290299 and THUMBDATA3-1763508120 files grow. They may become several hundred MBs, or in some cases, several GBs in size if you have a large number of thumbnail images in your Gallery app. The file increases in size every time a new image is added and even when an image is deleted the indexed properties of the image still remain.
To reduce the THUMBDATA3-1967290299 and THUMBDATA3-1763508120 file sizes, you need to remove images from the Gallery app then delete the THUMBDATA3-1967290299 and THUMBDATA3-1763508120 files. The files will be regenerated by the Gallery app but only for the images currently stored in the app.