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The idea is about to implement a two new places: Select window (or D&D) and actions.

Select window will view windows previews and allow to select one. File is saved to temp and KWin will (or other app) will uses D&D to send file onto selected window. There could be window atom telling that this window is this mechanism capable if D&D mechanism doesn't requires similar information.

Actions is quite different. Application could register actions and user could select action place in save dialog and select action. File dialog will save file and in next step program assigned to action will be executed with parameter points to path of stored file.

Fnotify could be useful - we watch for close events on file will path given to application, when user selects window/action.

Why?
Imagine I do some bank account operation and need to give info that operation is accepted by bank. Instead saving file, click attach in KMail, go to directory I've got write it and select it, I can simply select windows in save dialog, select "Composing a New Mail - Kmail2" window and click accept. That's it! Other possibility is to select action from places and compose a new mail with file as attachment. Very simple!