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There is no direct method to do this. I would do it with scripting and a third party service. It would require having both the long text field and a different attachment field. The script sends the long text to a third party service to be turned into a file, then the script turns the file into an attachment. Once all the files are attached, you can then delete the original long text field.
How many records do you have to convert? If it is less than a few dozen, you may want to just do it by hand. If you have several hundreds to thousands of records, I recommend the scripting method. If it is somewhere in between, it depends on you endurance for tedious tasks, your tolerance for accidental mistakes, your budget, and how soon you need the task done.