Oops! Terminal is opening all applications with texteditor after Open with:/Change All

Run plutil -convert xml1 ~/Library/Preferences/com.apple.launchservices.plist and add an entry like this under LSHandlers:

<dict>
    <key>LSHandlerContentType</key>
    <string>public.unix-executable</string>
    <key>LSHandlerRoleAll</key>
    <string>com.macromates.textmate</string>
</dict>

You can also change the default application for normal files with no extension:

<dict>
    <key>LSHandlerContentType</key>
    <string>public.plain-text</string>
    <key>LSHandlerRoleAll</key>
    <string>com.macromates.textmate</string>
</dict>

You can apply the changes by restarting. Just logging out and back in doesn't seem to work. You could also rebuild the Launch Services database, but it has the side-effect that the warnings about opening applications for the first time are shown again.

Another option is to add something like this to a duti configuration file:

com.macromates.textmate public.unix-executable all
com.macromates.textmate public.plain-text all

To make this question more searchable, here is the error message Finder shows when you try to change the default application for executable scripts:

The operation can’t be completed.

An unexpected error occurred (error code -50).

Okay, so you can do it for whatever Unix program you want to create a wrapper for.

Create an Automator application with a single action: Run AppleScript.

Make it run the following script:

on run {input, parameters}
        set x to "/usr/bin/vim " & quoted form of POSIX path of input
    tell application "Terminal"
        activate
        do script x
    end tell
end run

If you really want to use some other editor in place of vim, you could replace /usr/bin/vim in the second line of the script with the path to whatever command you are trying use to open files from Mail.