THM files are used by various programs for so-called thumbnail files - thumbnails are small images (thumbnail groups) that represent large images or video files in a summary view. These files are automatically created by various photo and video programs when browsing a picture or video folder for the first time, in order to be able to display or transfer the overview more quickly later (eg in online picture management programs).
Thumbnail files are usually compressed (JPEG) to save disk space and increase processing speed. However, most programs use their own containers, so the THM files can only be opened by the program with which they were created. However, most of them can be viewed in any image viewer, often the change of the file extension to jpg helps.
THM files can also represent videos, in which case the THM file is a copy of one of the first frames (video image). Each modern operating system also uses thumbnails for the thumbnails of picture folders. For example, Windows uses only one file per folder, the thumbs.Db.
Often THM files are also stored by video cameras on memory cards, in order to ensure a fast image preview for pictures or videos. So different cameras from Sony, Canon and Olympus use the extension thm. Depending on the transfer software but you get the THM file not to see.
Most often resolving problems with opening THM files is very simple. Just install an appropriate program that supports such files. All of the listed programs support THM files, but may vary in offered function and purpose. Some programs may be capable only of viewing contents of THM files or offer file conversion options, but may not be capable of editing such files.
It is possible that although a compatible program has been installed on user’s system, it is not used by the system to open it. This may happen because no default application was associated with this type of files. To associate THM files with given application, select "Open with" from drop-down menu (accessed by right-clicking on the file). Then select from the list the program or application you want to be used to open this type of file. If the Apple Preview applications is not on the list, select "Browse" option in order to manually locate the directory where Apple Preview has been installed.
Sometimes problems with opening THM files may not lie with the application itself, but can arise due to other causes, such as: