I think the problem is -- to Ubuntu, it doesn't seem that some of the files are executable at all or rather it doesn't know its file to type. The Files Manager due to which instead of showing the original icon (let's say the Arduino icon in this case) always tends to show some sort of unexecutable icon (as is visible in the attached image) for almost every (downloaded) file/program.
So, when I double click the file (say Arduino-ide) I get the message 'there is no application installed for "shared library" files.'
Note 1: I can mostly run the files/programs from the terminal with ./ and it works mostly correctly but that's not the permanent solution at all.
So, is it because some application/package is missing since I've done minimal installation of Ubuntu?!
Note 2: I've already tried chmod +x, chmod 777, and permissions tab of file's properties in Files (which probably uses chmod anyway) but nothing seemed to work.
Note 3: Also, for the (unreadable) icon problem, I've tried creating Desktop Entry (altering Exec, Path, Icon in the new, created text file) as was suggested in some of the other answers but that didn't work as well.
Here's the image 
Note 4: I know a few other similar questions exist but none of them have been able to resolve my situation as of now.
