Files open using wrong apps

I have a problem with my computer opening files using oddball applications. For example, I have instructed the computer to open all images using Photoshop--it keeps forgetting this and reverts to Adobe Illustrator. All text files are opening in a computer script program, even though I repeatedly "GET INFO" and tell the computer which application to use, then tell it to apply that rule to all similar files. Also, the names of images downloaded from the Internet are edited so that "picture.jpg" becomes "picture" and is not recognized as an image. I've made sure that my security settings are not preventing applications from opening.

Any ideas?

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