File formats
Why a Successful Migration Proves Nothing
A program can report on what it did. It cannot report on what you wanted, because it was never told.
OnlinePCApps Blog
Robert Smith
Senior File Format Engineer. Email and MIME parsing internals.
Showing all 39 articles.
File formats
How to Read a File Somebody Else Made
Removing something properly means rewriting the file. Marking it as gone costs nothing. Formats built decades apart made that trade for the same reason.
File formats
What to Do With Files Nobody Can Explain
Every individual step was sensible. None came with a note saying what it was, because writing that note was nobody job at the time.
File formats
What PDF/A Is And When You Actually Need It
A scanned page saved as PDF/A is a conforming file containing a photograph. It will look identical in fifty years and nobody will be able to search it then either.
File formats
What an Image File Remembers About Its History
A PNG showing block artefacts was a JPEG at some point. That is a reliable reading whatever the extension now says.
File formats
What a Signed PDF Actually Proves
The page is designed to look like paper and the proof is not on the page. That is the whole of it.
File formats
What a PDF Keeps After You Edit or Redact It
A black rectangle over text is a reasonable idea of what redaction means, and the format quietly does something else.
File formats
How a PDF Stores Text And Why Copying Goes Wrong
The page renders from the font and copying reads a separate mapping. Nothing forces the two to agree.
File formats
How Long Should You Keep Email
Storage is cheap and storage is not the cost. Everything kept is everything that can be asked for.
File formats
What Deleting an Email Actually Does
Almost every surprise here comes from somebody believing they did the fifth step when they did the second.
File formats
What Gets Lost When Email Changes Format
A run that moved everything and broke every attachment reports a perfect total. The count is the one thing that proves nothing.
File formats
How HTML Email Works And Why It Looks Different
Marketing email is built with tables and inline styles because that is the only construction that survives every client.
File formats
Why Email Dates Change After a Conversion
Half the mailbox an hour out and half correct reads like corruption. The split falls exactly on the date the clocks changed.
File formats
Which Email Formats Will Still Open in 20 Years
The people holding DBX files in 2003 had no reason to worry. It was the most common mail format on earth.
File formats
MBOX vs Maildir And the One Decision Behind Both
Maildir is better while mail is arriving. MBOX is better once it has stopped. That split is not accidental.
File formats
Why CSV Files Break And What They Never Tell You
A quoted value may contain a line break. It is also the one thing most simple parsers get wrong, and mail exports produce it as a matter of course.
File formats
CRLF vs LF And the Two Bytes That Break Things
The error message never mentions line endings. It names a file that plainly exists and appears to be spelled correctly.
File formats
Hashing vs Encryption vs Encoding And What Each Does
No key means no secret. If nothing had to be known to produce the result, nothing has to be known to undo it.
File formats
File Signatures And How Software Knows a Format
A wrong extension is a labelling mistake, not damage. The bytes were never touched and renaming costs nothing.
File formats
Character Encoding Explained And Why Text Arrives Broken
The bytes are almost always fine. What broke is the agreement about how to read them.
File formats
Online File Tools And Whether Yours Uploads
A privacy policy is a promise. A tool that finishes the job with the network switched off is a demonstration.
File formats
IMAP vs POP3 And Which One You Already Use
Most people never chose. A setup wizard chose for them. The consequences turn up when a laptop dies or an account closes.
File formats
EML vs MSG, One Holds What Travelled And One Holds More
The format that keeps more is also the one that fewer things can read. That trade is the whole decision.
File formats
Backup vs Archive And Why One Cannot Do Both
The organisation believed it had seven years of mail. It had fourteen days, repeatedly, for seven years.
File formats
What MIME Is And Why Every Mail Format Uses It
A handful of new headers, all of them plain text, all of them safe for a server that had never heard of them. It asked permission from nobody.
File formats
OST vs PST, One Format Wearing Two Contracts
The same structure on disk, holding the same things, under two different arrangements about who it belongs to.
File formats
Email Headers Explained And Which Ones to Trust
Received lines are written bottom upwards, so the oldest hop sits last. Read from the bottom and you are following the message forwards.
File formats
Why Email Attachments Are Encoded And What It Costs
A stated limit counts the message rather than the file. Twenty five megabytes of message holds about eighteen megabytes of anything you attach.
File formats
What is a Winmail.dat File And Why Only the Sender Fixes It
Outlook picks a message format per recipient rather than per message, which is why the same sender reaches ninety nine people cleanly and one badly.
File formats
What is a DBX File And Why Outlook Express Left Them Behind
A full DBX refuses new messages and refuses deletions too, so the first thing everybody tries is the one thing the file will not allow.
File formats
What is an MSG File And Why So Little Can Read One
Every value sits in its own stream named __substg1.0_0037001F or similar. Nothing in the file says which of those is the subject line.
File formats
What is an MHT File And Why Email Tools Produce Them
A saved page carries a Subject line and a sender because the container expects them. Neither means the file was ever sent anywhere.
File formats
What is an EDB File And Why It is Only Half a Database
Dirty shutdown reads like an error and is not one. Every database currently serving mail is in that state, which is why the header tells you so little.
File formats
What is an OST File And Why It Cannot Be Copied
The file copies perfectly. Outlook on the other machine is what refuses it, over one record written inside the file.
File formats
What is an OLM File And What is Actually Stored in One
Rename it to .zip and it opens. Inside, every message has been rewritten as XML, which is why getting mail back out is a rebuild.
File formats
What is an EML File And Why the Filename Matters
The format has no version and no variants. Everything awkward about EML comes from the folder around it rather than from anything inside the file.
File formats
What is an MBOX File And Why It is Just a Text File
Plain text, no index and no specification for its first thirty years. That history explains the four variants, the size on disk and why Outlook will not touch it.
File formats
What is a PST File And What Outlook Stores in It
Rules, views, categories and search folders all live in there beside the mail. None of them survive a move to any other format.
File formats
Every Email File Format Explained And Which One You Have
Two questions sort all of them. One message or a whole mailbox. Published structure or private. Everything else about portability follows from those two answers.
Reading because something will not open?
Every tool runs inside the browser tab, so a file is read where it already sits and nothing is sent to a server.