[Users] attachment 'open with' filepath screwed

[Michael]

Paul, many thanks ! This was indeed the problem.

I never ever would have guessed the spaces could be a problem. Weird.

Do we need the '%s' at all ? I mean, this is the 'open with' dialog, it should be a given that the command is used on that file. 

There might be rare cases where one would contruct a compliacted commandline, but then, does claws really need to care about those rare cases, especially when someone on that level, should be able to hack a more generally usable script, anyway. 

(For example, i keep such a 'toolbox' script for various desktop tasks, and spare to install and launch a dozen specialized apps, that way. One of the tradeoffs is that i can use it anywhere, MUA, file manager, text editors, image viewers ... whatever.)