What File Is This?

By reading its first bytes. Most formats begin with a fixed sequence called a file signature, or magic number, that the program writing the file always puts there: a PDF opens with the characters %PDF-, a PNG with an eight byte block starting 89 50 4E 47, a ZIP with PK. The extension is only a piece of the file name, stored by the folder rather than inside the file, which is why it can be missing or wrong without the content changing at all. The signature lives in the data itself, so it survives renaming, copying and mailing.

Usually because something along the way named the file rather than converted it. Mail clients and messaging apps rewrite attachment names, iPhone transfers hand out photos labelled .heic that are really JPEG, export dialogs let anyone type any extension, and file recovery tools produce thousands of files with generic names because the original ones were in the part of the disk that was lost. In every case the bytes are unchanged and correct: only the label is wrong, which is why the program you opened it with reported a corrupt file when the file was fine.

It means the file is a .p7m, a digital signature envelope, and the .pdf in the middle is the sender telling you what was put inside it. The last suffix is always the real container, and everything before it is part of the name. This matters because a signature envelope does not record the type of its payload anywhere in its own structure, so that middle suffix is the only clue about the content, and it is a claim by whoever named the file rather than a fact. It is also the single most misread file name in Italian office work: people see .pdf and expect a PDF reader to open it.

Find out what it is first, then give it the right extension or open it with a program that reads that format directly. Identifying it is the step that cannot be skipped, because the operating system chooses the program from the extension alone and has nothing to work with. Drop the file here: its signature names the format, this page offers the file back with the correct extension added, and it links the tool that opens it. Nothing is converted in the process, since the content was already in that format.

No, and confusing the two is the cause of a lot of wasted time. Renaming changes the label on the folder entry and nothing else: the bytes stay exactly as they were. If a file really is a PNG, renaming it to .jpg makes the name wrong and most programs will then refuse it or show an error. Conversely, if the content really is a JPEG and the name says .heic, renaming it to .jpg makes the name correct and the file will open. Renaming only helps when the name was the thing that was wrong.

The name can, the content cannot, and that gap is worth knowing about. A file called invoice.pdf.exe is a Windows program, because the last suffix is the real one, and Windows hides known extensions by default so it can appear simply as invoice.pdf. This is a classic delivery trick for malware in mail attachments. Reading the signature closes the gap: the bytes of an executable start with MZ no matter what the file is called, so an attachment that claims to be a document and reports as a program is answering your question.

No. The bytes are read by JavaScript inside your browser tab, matched against a table that ships with the page, and never sent anywhere. That is not a detail here: the files nobody can identify are usually the ones people are most careful with, like a recovered document, a signed contract or an attachment from an unknown sender. This page also works with the network disconnected once loaded, which is the simplest proof that no upload takes place.

The formats this site actually works with, which is the honest scope: PDF, the Office and OpenDocument families, Apple iWork, legacy Word, Excel and PowerPoint, email as EML, MSG, MBOX and winmail.dat, digital signature envelopes and certificates, Italian e-invoices, OFX and QIF statements, the common images including HEIC, AVIF, WebP and several camera RAW formats, the usual audio and video containers, the main archive formats, plus a few that are useful to name even though nothing here opens them, like SQLite databases and Windows executables. Text files are covered too, which is harder than it sounds because they carry no signature at all: XML, JSON, YAML, HTML, SVG, subtitles and delimited data such as CSV are recognised from the shape of their content, and a file that is plainly readable text with no structure to it is reported as exactly that. When a file matches nothing the answer is that it was not recognised, because a guess made from the extension would be a guess made from the exact field you came here to check.

Yes, completely free, with no sign-up and no file size limit. Only the beginning of the file is ever read, so the size of what you drop makes no practical difference to the speed of the answer.