FileLocator Pro can search in ANY file regardless of extension as long as the file has some sort of text in it. Some extensions, such as .ZIP, require special processing as they have a limited amount of plain text. FileLocator Pro contains 'interpreters' for many these common file types, e.g. ZIP, DOCX, XLSX, but can still search for plain text in other types, even .EXE and .DLL.
.BAK files are often created from text editors so I imagine that FLPro will have no problem with them at all.