There are hundreds of file formats and no tool opens all of them. What is achievable is one tool that opens the ones that actually reach you, and Doconut.app covers 70+ extensions in the browser — free, no account, deleted when you close the tab.

Universal File Support: Opening Any Document Instantly

The "Format Frustration"

We've all been there: you receive an email attachment, double-click it, and are greeted with the dreaded "Windows cannot open this file" message. Whether it's an obscure image format, a legacy word processing document, or a project file from software you don't own, format incompatibility disrupts workflow.

One Viewer to Rule Them All

Doconut.app removes that friction across seventeen viewers, each with its own size ceiling:

Office documents: Word, Excel, PowerPoint, and the OpenDocument, macro-enabled and template variants — 15 MB for Word, 20 MB for spreadsheets and decks.
Images: PSD, TIFF, WebP, SVG, EPS and camera raw (DNG) — 25 MB.
Technical: CAD drawings (DWG, DXF, DGN) at 10 MB, Visio diagrams at 25 MB, Microsoft Project plans at 20 MB, and email (MSG, EML, EMLX) at 10 MB.
Text and markup: TXT, XML, HTML and MHTML — 5 MB.

The ceiling belongs to the format, not to the site, so there is no single number to remember — the error message names the right one if you go over.

Where "universal" stops

Being accurate about the edges is what makes the rest of the list trustworthy:

It views; it does not convert. If you need the file in a different format rather than just readable, that is the sibling site OnlineDocumentViewer.com.
No editing, no annotating, no signing. The viewer draws the document; it never writes a new one.
One file at a time. No batch upload, no ZIP.
DICOM is excluded. .dcm and .ima are deliberately off the list for a free public tool, because a DICOM file carries patient identifiers with the image.
No OCR. Search works on a text layer the document already has, so a scanned page has nothing to find.

Within those edges the promise holds: you stop thinking about file extensions, and the attachment that used to stop you opens in a tab.

Start with the page for whatever is stuck — PDF, Word, Excel or CAD.