.BAK File

File extension bak is traditionally used for backups for decades.

Generally speaking, a bak file is a backup of original file, when the original extension is renamed to somename.bak. So, to restore original file, rename file to its original extension, if you know it, that is.

Many software apps also auto save with original documents or projects also a backup copy with the same file name.

How to open:

Unless you know the original file extension or software that created this bak file, is close to impossible to determine which program may open a *.bak file.

How to convert:

BAK files are often just renamed original files or previous versions of original file, so renaming the extension back to its original usually restores the functionalily.