What is the THUMBDATA3--1967290299 file format?
.thumbdata3--1967290299 is one of the .thumbdata3 format variants used by the Gallery and camera applications on Android phones. The file holds indexed properties and packed thumbnail images that the app uses to display previews quickly. The numeric suffix (-1967290299) is a per-device or per-folder hash, so the exact filename varies between phones - you may also encounter names like .thumbdata3-1763508120.
The purpose of a .thumbdata3--1967290299 file is to reduce load times by caching small JPEG preview images through an indexing mechanism. It is a proprietary binary cache that packs many tiny thumbnail JPEGs together with offset and index records - not a store for full-sized pictures. Because there is no official file signature, the internal structure is not formally documented, though embedded JPEG markers (FF D8 … FF D9) repeat throughout the file.
.thumbdata3--1967290299 files are created by the camera or gallery app to cache information about indexed images. The file size can vary from hundreds of megabytes up to several gigabytes, and it grows as more photos are added. To reduce its size, remove images from the gallery app and then delete the .thumbdata3--1967290299 file; a new file will be generated automatically, but only for the images that were not deleted.
.thumbdata3 files are saved in the sdcard/DCIM/.thumbnails/ folder and can be accessed using an Android file manager. Related Android cache formats include .thumbdata4 (the newer generation) and the .nomedia marker file, which tells the media scanner to exclude a folder from indexing.
Security & safety
RISK: LOWThe thumbnail cache is inert data and poses no risk to open, and it's safe to DELETE - your real photos and videos live elsewhere and aren't affected; Android simply rebuilds the cache. Two privacy notes worth knowing: (1) because the cache keeps thumbnails of deleted photos, it can retain low-res previews of pictures you thought you'd removed, so wipe it if you're handing the device on; (2) if a cloud backup uploaded this multi-GB file, it's just wasting backup space - you can safely exclude/remove it. There is no malware concern with the cache itself.
Format details
in a nutshellPrograms that open THUMBDATA3--1967290299 files
Windows2 apps
PhotoRec / a JPEG-carving tool (advanced)
Open-source Copy the file to a PC and run PhotoRec to carve the embedded thumbnail JPEGs out - last-resort recovery of low-res previews only.
Hex editor (HxD)
Free Open the file to confirm it's just packed thumbnails (you'll see repeated JPEG FF D8...FF D9 markers). Inspection only - there's no 'document' to read. 
Android2 apps
Any Android file manager (Files by Google, etc.)
Free Enable 'show hidden files', go to DCIM/.thumbnails (and Pictures/.thumbnails), and you can delete the .thumbdata3 file to reclaim space - Android rebuilds it automatically.
ZArchiver / a hex-capable tool (advanced)
Free Only for advanced recovery: the cache holds tiny embedded JPEGs; a carving tool can extract low-res previews. Not a normal way to 'open' it. Technical details
deep spec| Format type | Proprietary binary thumbnail cache - packs many small JPEG previews plus index/offset records into a single file |
| Platform | Android (generated by the stock Gallery or Camera app and OEM equivalents) |
| Developer | Google / Android AOSP; also OEM gallery apps (Samsung, HTC, etc.) |
| File location | sdcard/DCIM/.thumbnails/ folder on the Android device |
| Filename pattern | .thumbdata3--<hash> - the numeric suffix is a per-device/per-folder hash; varies between phones |
| File signature | No documented fixed header; embedded JPEG markers (FF D8 FF … FF D9) repeat throughout the binary content |
| MIME type | application/octet-stream |
| Encoding | Binary (not human-readable text) |
| Typical file size | Hundreds of MB to several GB; grows continuously as more photos are added to the device |
| Internal structure | Concatenation of small thumbnail JPEGs separated by index and offset records used by the Gallery for fast lookup |
| Thumbnail dimensions | Typically 96 × 96 px (micro) or 512 × 384 px (mini), matching Android MediaStore thumbnail specs |
| Cache purpose | Speeds up Gallery preview loading - pre-built index avoids rescaling source images on every gallery open |
| Lifecycle | Auto-created by the Gallery app; safe to delete - the app regenerates it on the next media scan |
| Thumbnail extraction | JPEG thumbnails can be carved from the file using PhotoRec or a similar JPEG-carving tool |
| Related formats | .thumbdata (earlier generation), .thumbdata4 (newer generation), .db (MediaStore SQLite), .nomedia (scan exclusion marker) |
| Released | Android (Gallery/Media thumbnail cache); the 'thumbdata3' generation has been around since the early 2010s |
| Specification | fileinfo.com |
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