What I experienced in past that mime type (for static resources) depends on the browser used to upload the file. I got the below mime types for same javascript file from different browser.
Chrome - application/javascript
Firefox -application/x-javascript
IE 11- application/octet-stream
So you can try to upload the file from chrome and see if you get application/javascript as mime type.
Or else you can use Eclipse IDE to change the mime type. Change the MIME type in the static resource's meta XML file from application/x-javascript to application/javascript.