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From MoodleDocs

One reason for this error is having an apostrophe in your file name like "Joe's file". When uploading such a file it replaces it sometimes with a single letter instead of your title. Then if you try to rename it with an apostrophe, it will fail to choose it as a resource and if you click on the resource to open it from your files it will return this error instead of opening. Taking the apostrophe off fixed my issue with the problem. Good luck moodling, Jamie Tinley.

Moodle also doesn't like spaces within file names. So changing "this is my text.pdf" to "this_is_my_text.pdf" solves this problem.