I installed Wine on an Ubuntu Linux system to be able to run Microsoft Windows applications on it. I installed Sumatra PDF to view PDF and EPUB files. When I double-clicked on EPUB files in the Wine File Explorer window, which you can run by issuing the command wine explorer in a Terminal window, or wine explorer &, if you wish to open it and immediately return to the shell prompt, I saw an Error window which stated "There is no Windows program configured to open this type of file." If I double-clicked on a PDF file, it would open in the GNOME Document Viewer application. I was able to set the default application for opening those files under Wine with the following steps:

1. From the Bash prompt in a Terminal window, run the Wine registry editor program to modify the Windows Registry with wine regedit.
2. Navigate to HKEY_CLASSES_ROOT\.epub and set (Default) = epubfile. If you don't see a .epub entry, right-click on HKEY_CLASSES_ROOT and choose New. You can press the F5 key or choose View and Refresh to refresh the display to show the new entry in alphabetical order.
3. If you then click on the .epub entry, you will see (Default) under Name, REG_SZ under Type, and (value not set) under Data.
4. Double-click on the entry and then type epubfile in the Value data field and then click on OK.
5. Then navigate to HKEY_CLASSES_ROOT\epubfile\shell\open\command, if it exists. If there is no epubfile entry you can create one by right-clicking on HKEY_CLASSES_ROOT and selecting New and Key giving the new key a name of epubfile. Then double-click on the (Default) entry under the new key and type EPUB Document in the Value data field. Then right-click on the epubfile key and choose New and Key and type shell for the name. Right-click on shell and create a new, open beneath it. Then right-click on open and create a new key, command beneath open.
6. Then right click on the command key and choose New then String Value.
7. Double-click on the new (Default) entry under command and then type the location and file name for the application that you wish to use to open .epub files within double quotes followed by a space and then "%1", e.g. "C:\Program Files\SumatraPDF\SumatraPDF.exe" "%1", if you wish to open the files with Sumatra PDF and you installed it so that it is accessible to all users of the system. Or use "C:\users\username\AppData\Local\SumatraPDF\SumatraPDF.exe " "%1", where username is the relevant username, if you chose to install it just for the current user when you installed it.

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