File tools
Browser-side file portability and inspection tools — "will this look the same everywhere, and why?"
Markdown Portability Preflight
Finds Markdown constructs that render differently across GFM, CommonMark and stripped-down renderers.
EML/MIME Import Doctor
Checks a raw .eml file for constructs that behave differently across mail clients and importers.
Text File Portability Preflight
Checks a plain text file for encoding/line-ending/whitespace issues that behave differently across operating systems, editors and tools.
MIME Attachment Filename Doctor
Checks attachment filenames declared inside a raw .eml/MIME message for portability problems when saved to disk across Windows, macOS and Linux.
Why files look different in different places
A file that opens perfectly on one system can break on another because of details you can't see: line endings (Windows CRLF vs Unix LF), a hidden byte-order mark, a character-encoding mismatch, or an attachment filename encoded in a way an email client doesn't understand. These tools inspect those details and explain what they found.
Pick the right tool
For plain text files, start with the text file portability preflight. For Markdown that renders differently between platforms, use the Markdown portability preflight. For saved emails that won't import, or attachments with garbled names, use the EML/MIME import doctor and the MIME attachment filename doctor.
Files are read locally
Selected files are read by your browser and never uploaded. Each check covers the specific issues its page documents; a clean result doesn't guarantee every application will handle the file identically.