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Currently, the cms.plugins.file.models.File file upload field has no help text at all. As no max length has been declared, the field's default maximum string length is 100 for all database backends. Ideally it make sense to provide help text that thus documents the maximum filename length for any upload (minus the media path), but as get_media_path is a function that acts on a filename, I'm not sure how viable it is. Consider this ticket me asking cleverer people than I if it's plausible; if not, close as invalid/wontfix.
The actual filename of the uploaded file is not necessarily related to the length of the string stored in the field if you use a non-standard file backend (eg: S3). Therefore I'm not sure we can come up with a reasonable generic help text.
file plugin removed from core