What causes "validation_error_filename" in instructure/canvas-lms?

Replace the filename with a human description of the image's content

instructure/canvas-lms · error · StandardError

validation_error_filename

Error message

validation_error_filename

What it means

ImgAltRuleHelper rejects alt text that looks like a filename (via filename_like?). Filenames such as 'image.png' or 'IMG_1234.jpg' are not meaningful alt text, so the accessibility rule raises StandardError with the validation_error_filename message to force the author to describe the image instead.

Solutions

Replace the filename with a human description of the image's content
Adjust input so the user cannot submit while the value matches the filename heuristic
If the heuristic false-positives on legitimate text, rewrite the value so it doesn't match the pattern (e.g. remove extension-like suffix)
Rescue the error and re-prompt the user for a description

Example fix

// before
fix(elem, 'IMG_2049.jpg')
// after
fix(elem, 'Students collaborating in a lab')

Defensive patterns

Strategy: validation

Validate before calling

const FILENAME_RE = /\.[a-z0-9]{2,4}$/i;
if (FILENAME_RE.test(alt_text.trim())) {
  alert('Describe the image, do not use the file name'); return;
}
submitFix(elem, alt_text)

Type guard

const isDescriptive = (v) => typeof v === 'string' && v.trim().length > 0 && !/\.\w{2,4}$/.test(v.trim())

Try / catch

begin
  helper.fix(elem, value)
rescue StandardError => e
  flash[:error] = e.message
  redirect_back
end

Prevention

Never prefill alt fields with the uploaded file's name
Warn inline as the user types if the value matches filename heuristics
Educate authors that alt text describes content, not origin

When it happens

Trigger: Calling the img alt fix with a value that filename_like? classifies as a filename (e.g. contains extensions like .png/.jpg, CamelCase-with-digits patterns); user pastes the uploaded file's name into the alt field.

Common situations: Course authors leaving the default filename in the alt text box; bulk migration scripts copying title attributes containing file names into alt; CMS imports preserving original upload names.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of instructure/canvas-lms@1c9f0bb801 (2026-09-15). Data as JSON: /api/errors/6066534cb91f885f. Report an issue: GitHub.

Appendix: source

Thrown at app/models/accessibility/rules/img_alt_rule_helper.rb:65

      end

      def self.validation_error_too_long
        I18n.t("Keep alt text under %{max_length} characters.", max_length: MAX_LENGTH)
      end

      def self.fix_alt_text!(elem, value)
        if value.nil?
          elem["role"] = "presentation"
          elem["alt"] = ""
          return { changed: elem, content_preview: adjust_img_style(elem) }
        end

        if value.to_s.strip.empty?
          raise StandardError, validation_error_missing
        end

        if filename_like?(value)
          raise StandardError, validation_error_filename
        end

        if value.length > MAX_LENGTH
          raise StandardError, validation_error_too_long
        end

        elem["alt"] = value
        { changed: elem, content_preview: adjust_img_style(elem) }
      end
    end
  end
end

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