I only found information to add the “send attachment” entry to the right-click menu for Thunderbird and Outlook. Both require almost the same steps only the command that is used will be different. Open the registry by pressing Windows + R, typing regedit and hitting enter. Navigate to the key Hkey_Classes_Root \ * \ shell. If the shell entry is not there create it by right-clicking on * and selecting New > Key from the menu and name it shell.

Now create another key in the shell key and name it any way you like. This will be shown in the right-click menu so choose something like “send attachment” for instance. Create another key in the newly created key “send attachment” and name it “Command”.

Right-click on the Default entry in the right pane of Command and right-click it. Select modify from the menu and add the following entry if you want to send attachments to Outlook: "Path to Outlook.exe" /a "%1"

If you want to send attachments with Thunderbird change the default value to: "Path to Thunderbird.exe" -compose attachment="file://%1". My command looks like the following to give you an impression "C:\Program Files\Mozilla Thunderbird\Thunderbird.exe" -compose attachment="file://%1"