What Is an APPLICATION File?
/A file with the .APPLICATION file extension is a ClickOnce Deployment Manifest file. They provide a way to launch Windows applications from a web page with just one click.
APPLICATION files hold information about application updates by including the name, identity of the publisher, application version, dependencies, update behavior, digital signature, etc.
Files with the .APPLICATION extension are seen alongside .APPREF-MS files, which are Microsoft Application Reference files. These files are what actually call on ClickOnce to run the application—they hold the link to where the application is stored.
How to Open an APPLICATION File
APPLICATION files are XML-based, text-only files. This means that Microsoft's Visual Studio or even any text editor should be able to properly read the file. There are several free text editors that will do the job nicely.
.NET Framework is required to actually run .APPLICATION files.
ClickOnce is a Microsoft system—they have more information about this type of file through that link. Technically, the Microsoft ClickOnce Application Deployment Support Library is the name of the program that opens .APPLICATION files.
It's likely that ClickOnce will only open if the URL is accessed through one of Microsoft's browsers: Edge or Internet Explorer. This also means that programs like MS Word and Outlook can open the .APPLICATION file only if Microsoft's browser is set as the default browser.
Sometimes regular document, music, or video files are incorrectly referred to as application files, like .PDF, .MP3, .MP4, .DOCX, etc. These file formats have nothing to do with the .APPLICATION extension.
How to Convert an APPLICATION File
You should be able to open an .APPLICATION file in Visual Studio and then save the open file to another format. Of course, XML editors can also save .APPLICATION files to some other format.
However, keep in mind that changing the format to something else means that anything that relies on the .APPLICATION file to function will no longer work as it should in the new format.
Still Can't Open the File?
Other files formats may use a similar file extension, but they really have nothing to do with ClickOnce Deployment Manifest files. If you can't open your file, re-read the file extension to see if you're really dealing with a totally different file format.
For example, APP files may be macOS or FoxPro application files, and APPLET files are used by Eclipse as Java Applet Policy files. APK is another file extension that might be confused for an APPLICATION file.
This article and cited credit goes to:
https://www.lifewire.com/application-file-2619655