THM File Documentation

Summary

A file with the .thm extension is almost always a Thumbnail Image File: a small JPEG preview that a digital camera, camcorder or GoPro automatically saves next to a video or photo of the same base name (for example GX010123.MP4 gets a GX010123.THM). Despite the extension it is an ordinary JPEG — it begins with the JPEG start-of-image marker FF D8 FF and has MIME type image/jpeg. Rename it to .jpg and any image viewer opens it. THM files are previews only and can be deleted without affecting the video.

Technical details

Feature	Value
Full name	Thumbnail Image (camera / camcorder video thumbnail)
File extension	`.thm`
MIME type	`image/jpeg`
File type	JPEG image (despite the `.thm` extension)
Developer	Various camera makers (Canon, GoPro, Sony, Olympus, Panasonic)
Introduced	Long-standing DSLR/camcorder convention; common on GoPro since the 2010s
Compression	Lossy JPEG (DCT)
Colour depth	24-bit (8 bits per channel, YCbCr)
Typical dimensions	Low-resolution preview, commonly around 160×120
Typical file size	Small, often under 10 KB
Magic number	`FF D8 FF` (JPEG SOI marker) at offset 0
End marker	`FF D9` (JPEG EOI)
Metadata	Optional JFIF/EXIF; some cameras copy EXIF from the source clip
Standalone usability	Yes as an image; not required to play the associated video
Safe to delete	Yes — it is only a preview
Category	Raster Image Files
Related extensions	`.jpg`, `.jpeg`, `.lrv`, `.mp4`, `.mov`, `.cr2`

File signature (magic bytes)

FF D8 FF E0

Offset 0. A camera .thm is a JPEG, so it starts with the JPEG Start Of Image marker FF D8, immediately followed by an application marker: FF E0 introduces a JFIF APP0 segment, or FF E1 an EXIF APP1 segment when the camera copied metadata from the source clip. The file ends with the End Of Image marker FF D9. Because the bytes are identical to any JPEG, renaming .thm to .jpg lets any image viewer open it. If the first bytes are not FF D8 FF, the file is not a camera thumbnail but one of the rare legacy uses of the extension.

What is a THM file?

THM is short for thumbnail, and a .thm file is overwhelmingly a small JPEG preview image generated automatically by a digital camera, camcorder or action camera. When the device records a video (or, on some cameras, a raw photo), it writes a small JPEG alongside the clip with the same base name and a .thm extension — Canon writes MVI_1234.THM next to MVI_1234.MOV, and a GoPro writes a .THM next to each clip. The camera and its import software use this thumbnail to show a fast preview in the playback menu or the offload app without decoding the full-resolution video.

Despite the unusual extension, the file is a normal JPEG image, typically low-resolution (commonly around 160×120). Its MIME type is image/jpeg. That single fact drives almost everything useful about the format: because it is a JPEG, any image viewer opens it once it is renamed to .jpg, and because it is only a preview, it is never required to watch or edit the associated footage.

The anatomy of a THM file: it is a JPEG

Structurally a .thm is identical to a standard JPEG. It is a sequence of marker segments, each introduced by a byte pair beginning with 0xFF, wrapping DCT-compressed image data.

FF D8              SOI   Start Of Image
FF E0 00 10 4A 46 49 46 00 01 ...   APP0 (JFIF header)  -- or
FF E1 .. .. 45 78 69 66 00 00 ...   APP1 (EXIF header, when present)
FF DB ...          DQT   quantisation table(s)
FF C0 ...          SOF0  start of frame: dimensions, components
FF C4 ...          DHT   Huffman table(s)
FF DA ...          SOS   start of scan: the compressed pixel data
FF D9              EOI   End Of Image

The file opens with the Start Of Image marker FF D8 and closes with the End Of Image marker FF D9. Immediately after SOI comes an application segment: a JFIF APP0 (FF E0) on plain thumbnails, or an EXIF APP1 (FF E1) when the camera copied metadata across from the source clip. The image itself is 24-bit colour (8 bits per channel in YCbCr) compressed with the usual JPEG DCT and Huffman coding. There is nothing camera-specific about the pixel encoding; the only thing non-standard is the extension on the filename.

Because the format is exactly JPEG, a .thm does not contain the video’s codec, duration or frame rate as structured fields. At most it carries an EXIF block copied from the still-image pipeline. The thumbnail is a picture, not a metadata sidecar for the video, and it is not guaranteed to be the video’s literal first frame — it is simply a representative low-resolution preview the camera chose to write.

Viewing it: rename to .jpg

Since the bytes are a JPEG, the reliable way to view a .thm is to copy it and change the extension from .thm to .jpg, after which any viewer — Windows Photos, macOS Preview, a phone gallery — opens it as an ordinary image. Some image tools (IrfanView, XnView MP) read .thm directly without renaming, because they detect the FF D8 FF signature rather than trusting the extension. This is not a conversion in any real sense: the file already is a JPEG, and renaming changes only the label the operating system uses to pick a default program.

The GoPro THM/LRV trio

GoPro cameras are the most common reason people encounter .thm files today, and GoPro writes not one but two helper files per clip. For each recording you get three files sharing a base name:

File	What it is
`GX010123.MP4`	The full-resolution video — the only file you need to keep
`GX010123.LRV`	A Low Resolution Video proxy, used for fast preview and mobile editing
`GX010123.THM`	A small JPEG thumbnail of the clip

The .LRV is itself a real video file (an MP4-family low-bitrate proxy), which is why it can be renamed to .mp4 and played, while the .THM is a still image. Both the .THM and .LRV exist purely to make browsing and editing fast; the GoPro app uses the proxy so it does not have to decode the full 4K stream while you scrub. Neither is part of the final footage.

Can you delete THM files?

Yes, safely. A .thm is only a preview thumbnail; deleting it does not touch the video, which plays and edits exactly the same without it. The same is true of GoPro .LRV proxy files — you can remove them too, at the cost of slower previews in the GoPro app until it regenerates them. The only thing that matters for your final result is the full-resolution .MP4/.MOV/.AVI clip. Some editing applications use the .thm for quick browsing, so if you keep footage in an editor’s media library you may prefer to leave the thumbnails in place, but nothing breaks if they are gone.

Legacy and other meanings of .thm

A small number of unrelated things have historically used the .thm extension, and they are worth knowing about only so you can rule them out. Some 2000s Sony Ericsson feature phones used .thm for a phone theme package, which is not an image at all. A few older clip-art galleries and applications used a .thm thumbnail-cache database. If a .thm file does not begin with the JPEG signature FF D8 FF, it is one of these non-image legacy types rather than a camera thumbnail, and renaming it to .jpg will not help. For any .thm you actually find sitting next to video clips from a camera or GoPro, though, the JPEG-thumbnail meaning is the correct one.

FAQ

Is a THM file an image or a video? An image — a single still JPEG thumbnail. The video is the separate file with the same base name and a video extension (.MP4/.MOV/.AVI). A .thm cannot be played as video; on GoPro the low-resolution video proxy is the .LRV, not the .THM.

Why won’t my .THM file open? Most likely no program is associated with the extension. Because the file is a JPEG, rename it to .jpg and it opens in any image viewer. If it still will not open as an image, check the first bytes: without the FF D8 FF signature it is a different, non-image legacy file type that happens to share the extension.

References

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