For example I cannot drag and drop folders and .py files to the meld using launcher but it accepts them perfectly.
Locate the .desktop of app you wish to adjust, in the case meld & open in a text editor.
Usually there will be a line, MimeType=, just add to it the appropriate type, if you don't know then a right click > Properties > Type will show.
So in this case, 'Meld', you'd add this to the end of the MimeType= line
If for some reason the line didn't exist the just add it, like
Usually a logout/in will be needed to reflect changes.
For meld - try also editing the Exec= line in the .desktop to this
Other possibilities for letter would be f or U, though glancing at the report I'm not sure it will matter much here with meld