To open a .THM file on a PC or a Mac, an image preview or image editing program can be used. A .THM file is a file format used for image thumbnails and is smaller than the original image file to reduce loading time.
A file with a .THM format is normally automatically generated when a video file is extracted from a camcorder. Thumbnail files are closely associated with the video files that they are generated from and will have the same name. A file in a .THM format can be changed to a .JPG or .JPEG file by renaming the file's extension.