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Assuming there are two different dictionaries, A and B, on the computer, should I use dictionary or dictionaries in the compound noun dictionary files in the following sentence? I believe it should be something like ''dictionaries' files'', not dictionary files because there are two different dictionaries kept in two different folders.

These dictionary files were not deleted along with other files because these were in use at the time of deletion.
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Hello Jackson,

I would agree with Marius – "dictionary" here modifies "files" in an adjectival fashion.

All the best,

MrP

PS: I like your Titian.
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Why?
 MrPedantic's reply was promoted to an answer.
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Using plural nouns as adjectives is generally avoided, though possible.
You're talking mainly about the type of files, not about their multitude, thus indicating that they are in several places is less significant.