Refactor `ArtifactPlugin.read_file()` and `FileWidget._open_file()`

Refactor ArtifactPlugin.read_file() and FileWidget._open_file() as suggested in !2284 (comment 690040)

(ArtifactPlugin.read_file() exists when merging !2284 (merged) which refactored it to use it from different plugins)

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Refactor ArtifactPlugin.read_file() and FileWidget._open_file()