OpenAnyFile
What is a .master file?
ASP.NET Master Page is a source code format developed by Microsoft, first released in 2005. It is usually created by ASP.NET Web Forms.
A shared layout for .aspx pages.
File details
| Full name | ASP.NET Master Page |
|---|---|
| Category | Code |
| MIME type | text/plain |
| Developer | Microsoft |
| Introduced | 2005 |
| Opens with | Code viewer |
| Uploaded? | No — read locally in your browser |
How to open a .master file
OpenAnyFile routes .master files to its code viewer, which shows the source with syntax highlighting.
- Drop your .master file on the box above, click it to choose the file, or paste it with Ctrl+V.
- The real format is detected from the file's first bytes — not just its name.
- The file opens in the code viewer.
- Use fullscreen for a closer look, check its checksums, or download it back unchanged.
What works — and what does not
Works
Viewing and inspecting: the source with syntax highlighting. Size, type, signature and checksums are always shown.
Does not work
Editing and saving back. OpenAnyFile is a viewer, not an editor — for changes you still need the original app.
Is it safe to open a .master file online?
Here, yes — because nothing is uploaded. Most "online viewers" send your file to their server. OpenAnyFile reads it with your browser's File API and decodes it on your own device, so there is no copy anywhere else. You can check: open your browser's network tab while opening a file — no upload appears.
Questions about .master files
Can I open a .master file without installing software?
Yes. OpenAnyFile opens .master files in your browser using its code viewer. You get the source with syntax highlighting, with nothing installed and no account.
How do I know a file is really a .master file?
Check its first bytes instead of its name. This format has no fixed signature, so the detected type and the readable content are the best clues. OpenAnyFile shows the detected type, so a renamed file is easy to spot.
Is my .master file uploaded anywhere?
No. The file is read locally by your browser and never sent to a server — it even works offline once the page has loaded.
Does it work on phones?
Yes — on Android and iPhone, in any modern browser. You can also install OpenAnyFile as an app.
Related formats
Other formats from the same family:
Format facts checked and page updated on September 25, 2026.