What is the ATT file format?
.att is a data file format used for storing web form data. .att files contain data from filled web forms - specifically the field=value pairs a browser submits to a server when a user completes and sends an HTML form.
The classic filename for this format is postdata.att. An .att file is generated when the contents of a web form are submitted by a user and the form has an incorrect or missing encoding configuration. In that situation, instead of being processed server-side, the data is packaged and sent to the recipient via email as an attachment named postdata.att.
.att files store data in plain text format, and their structure is similar to a CSV file - each form field and its submitted value appear as key=value pairs, making the content directly readable in any text editor without special software. Data fields from an .att file can be imported into an Excel spreadsheet for further analysis.
No single vendor or standard defines the .att format; it is an informal by-product of how web browsers and servers handle form submissions when the Content-Type encoding is absent or misconfigured. It carries no binary file signature - opening it in a text editor shows the readable form data directly. The .att attachment typically arrives inside an EML or MSG email message file.
Note that the .att extension is also reused by unrelated programs for entirely different file structures; those are separate file types with no connection to web form data.
Security & safety
RISK: LOWA postdata.att web-form file is plain text and harmless to open. The genuine caution is social-engineering: an unsolicited email attachment named "att" or "postdata.att" could be bait - confirm it's actually text before trusting it, and never run anything it asks you to. Make sure the file is really plain text and not an executable renamed to look like a harmless attachment (check the true type).
Format details
in a nutshell- Alphacam lathe-tool data - Alphacam CAD/CAM software saves lathe tool definitions as .att, opened by Alphacam itself.
- Creatures game body data - The Creatures life-simulation games use .att for body-part attachment data that maps how breed sprites connect.
- Secure/e-mail attachment payload - Some mail systems have used .att for attachment payloads, occasionally encrypted.
Programs that open ATT files
Windows2 apps
Windows Notepad
Built-in Right-click the .att → Open with → Notepad. You'll see the field=value text directly. 
Notepad
Open-source Open the .att to read/edit the text; handles encodings and large files better than Notepad. 
macOS1 app
TextEdit
Built-in Open the .att in TextEdit (choose plain-text/UTF-8 if prompted) to read the web form data. 
Linux1 app
Any text editor (gedit, nano, VS Code)
Open-source Open the .att as plain text to read the field=value submissions. Technical details
deep spec| Format type | Plain text; sequential field=value pairs from an HTML form submission |
| MIME type | text/plain |
| Canonical filename | postdata.att - the filename is a near-universal identifier for this specific file type |
| File signature | None - no binary header or magic bytes; content is immediately human-readable as plain text |
| Character encoding | Matches the source HTML form; typically US-ASCII or UTF-8; special characters may be percent-encoded |
| Data structure | One key=value pair per line corresponding to each named HTML form field |
| Value encoding | Special characters and spaces within field values are URL-encoded (percent-encoding, e.g. %20 for space) |
| Trigger condition | Produced when a web form with a mailto: action is submitted and the browser wraps the submission data as a form-data attachment |
| Delivery method | Arrives as an email attachment, typically inside an .eml or .msg message file |
| Spreadsheet compatibility | Field data can be imported into spreadsheet applications such as Microsoft Excel by treating it as delimited text |
| Developer / vendor | No single owner - an informal artefact of the HTML form submission mechanism; no formal specification exists |
| Line ending | Platform-dependent: CRLF on Windows-origin submissions, LF on Unix/Linux servers |
| Openability | Fully human-readable in any plain-text editor: Notepad, TextEdit, VS Code, nano, gedit, and others |
| Related formats | Structurally similar to plain .txt and CSV files; typically delivered inside an .eml or .msg email message |
ATT conversions
Community Q&A
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