If you received a file with unknown extension, how will you find the proper application to open it ? you will have a search and may or may not find an application for the purpose, it may be some costly applications sometimes.
The OpenWith Desktop application will allow you to find what is the file type, and which Free programs can open it. This application will also provide a download link for the application too, and will allow you to download with out visiting their website or download location.
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Right click on the file with unknown file extension and choose OpenWith.org – How do I Open This from the menu
The application will show the applications which can open this kind of files with a download link (If a supported application is installed on your system it will show as installed)