The .slm file extension can be linked to files being generated when the user utilizes the Gallery Lock. Gallery Lock manages and stores pictures and videos by hiding them to other users. Developed by Morrison Software, Gallery Lockis an app for Android that allows users to hide photos and other pictures on their smartphones.
The .slm file is an encrypted/ encoded file type that represents sthe altered version of the original "hidden" picture which is located in a hidden folder on the SD card in the following location: /mnt/sdcard/.gallery_lock/. Basically, 1024 bytes of data is added to the image to put it into hiding and the extension is renamed from .jpeg (for example) to .slm. It, of course, uses password protection to be able to unhide and open the file.
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If you have additional information about the SLM file format or software that uses files with the SLM suffix, please do get in touch - we would love hearing from you.