What is the THUMBDATA3--1763508120 file format?
Files with the .thumbdata3--1763508120 extension contain photo thumbnail images generated by older Android Gallery applications. .thumbdata3--1763508120 files are produced by the Android Camera or Gallery app as index files that map cached thumbnail slots to the original media items. The size of the file increases as the photo collection grows larger, and entries for pictures that were later deleted may still remain indexed - the cache does not remove stale entries automatically.
The .thumbdata3 format's purpose is to speed up image browsing. Instead of opening large originals (which can be several MB each) every time the user browses the photo collection, the Gallery reads from the index, which holds small JPEG thumbnails typically only a few kilobytes each. The file is a proprietary binary blob: many compressed JPEG previews are stored back-to-back inside the container, so raw JPEG SOI markers (FF D8 FF) appear at multiple internal offsets rather than at the very start of the file.
The trailing signed integer in the filename - here -1763508120 - is a per-device, per-version hash tag generated by the gallery app and varies across Android devices and builds.
Thumbnail cache files are generated automatically and in extreme cases can occupy a large amount of storage space. .thumbdata3--1763508120 can be safely deleted to reclaim space; however, upon restarting the application that generated it, the file will be recreated. Because the cache does not update its contents to account for deleted images, the only way to refresh the index is to delete the file and let Android rebuild it. The cache is stored by default in the hidden folder /DCIM/.thumbnails/ on the device storage, accessible by enabling hidden file visibility in a file manager.
The .thumbdata3 variant is associated with Android 4.x-era devices. Newer Android versions use .thumbdata4 or .thumbdata5 cache files instead. A sibling .nomedia file in the same folder prevents Gallery apps from indexing the thumbnails themselves as part of the photo library.
Security & safety
RISK: LOWThe file itself is harmless passive cache data - it cannot execute. Two real cautions: (1) Deleting it is safe and does NOT delete your photos, but some users panic and delete the wrong thing - only the .thumbdata cache in .thumbnails is expendable. (2) Privacy: because it holds previews of every photo the gallery ever indexed, the cache can contain thumbnails of pictures you already deleted; if you hand a phone or its backup to someone, wipe this file. 'Thumbnail recovery' is also a known forensic technique - the cache can reveal deleted images.
Format details
in a nutshellPrograms that open THUMBDATA3--1763508120 files
Windows2 apps
PhotoRec
Open-source Copy the file to a PC and let PhotoRec carve out the embedded JPEG thumbnails (low-res previews only).
Any hex editor
Free Open in HxD only to confirm it's a thumbnail blob - search for repeated FF D8 FF JPEG headers. Not a real 'viewer'. 
Android2 apps
Delete via Files by Google (recommended action)
Free Enable 'show hidden files', go to DCIM/.thumbnails, delete .thumbdata*; the gallery rebuilds it. There is no reason to 'open' it.
Any JPEG-carving / thumbnail recovery app
Open-source Only if recovering lost previews: run a JPEG carver (e.g. PhotoRec) over the file to extract the embedded thumbnail JPEGs. Technical details
deep spec| Format type | Proprietary binary thumbnail cache - concatenated JPEG thumbnails plus an undocumented index region in a single blob |
| MIME type | application/octet-stream |
| Encoding | Binary blob; no text encoding; raw JPEG thumbnail data packed back-to-back with no standard framing |
| Internal compression | Each embedded thumbnail is JPEG-compressed; the outer container itself is uncompressed |
| Typical file size | Tens of MB to several GB, growing proportionally with the number of indexed media items |
| Thumbnail dimensions | Commonly 256-512 px on the long edge per individual thumbnail |
| Color depth | 8-bit per channel, 24-bit RGB - standard JPEG color space for each embedded preview |
| Magic bytes | No stable magic number at offset 0; JPEG SOI markers (FF D8 FF) appear at multiple internal offsets where individual thumbnails begin |
| Filename hash suffix | The trailing signed integer (e.g., -1763508120) is a per-device, per-version hash generated by the gallery app; it varies across devices and Android builds |
| Default storage path | /DCIM/.thumbnails/ as a hidden folder on the device's internal storage or SD card |
| Associated OS / era | Android 4.x-era devices running the stock Gallery or Camera app |
| Superseded by | .thumbdata4 and .thumbdata5 cache formats introduced in later Android versions |
| Deletion behavior | Safe to delete to reclaim storage; the Gallery app automatically rebuilds the cache on next launch |
| Stale entry handling | Deleted photos are not purged from the index automatically; rebuilding requires deleting the file and letting Android regenerate it |
| Sibling file | A .nomedia file in the same /DCIM/.thumbnails/ folder prevents gallery apps from treating the cached thumbnails as user photos |
| Released | Android gallery thumbnail caching (stock Gallery); '.thumbdata3--' variant seen on Android 4.x era devices |
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