.NOMEDIA is a one of the special file formats used by ANDROID operating system. .NOMEDIA files are found in various folders located on storage devices managed by ANDROID. Such files can also be found on memory cards and other similar storage devices. .NOMEDIA files are only meaningful to Android operating system.
.NOMEDIA file purpose is to tag a folder. Each folder that contains a file with NOMEDIA file extension is excluded from multimedia scanner indexing results. This means that any multimedia files stored is such folder will not be visible in such applications as image galleries or multimedia players.
Contents of file with .NOMEDIA extension
By default files with .NOMEDIA file extension do not store any data, either binary or in plain text. Also, .NOMEDIA files have no base name by convention (they use the generic .NOMEDIA tag). System does not check contents of .NOMEDIA files, only the presence of it in the folder is important for the indexing exclusion to work properly.
User used this fact to hide some vital data by changing sensitive file name to .NOMEDIA and storing such file in system-like folders. This makes harder for unprivileged users to access such data as they must peruse multiple .NOMEDIA files with hopes of finding the right one(s).