No install No upload No analytics on this page
Try it on a file, right here
Four things Foretaste does, running in this tab and nowhere else: the workflow on a page full of file links, what a name alone can tell you, what the bytes of a file of your own say it is, and an archive on this server listed without being downloaded.
Every one of these is the real code. The detector is the extension's own lib/magic.ts; the archive is read over real HTTP range requests, and your browser's network panel will show you every one of them.
Demo 01
The workflow, on a page of links
Right-click a file name and choose Open with Foretaste, or press the marker beside it.
Downloads / 2026 / Course materials
Autumn term: files
Everything for the autumn term is below. The archive has the slides and the readings in it; the rest are loose.
- ZIP · 40 MB
- no extension
- XLSX · 84 KB
- SQLite
Press a marker to see what the file is, or right-click a name.
course-materials.zip
ZIP archive
The page says 40 MB
Nothing has been fetched yet.
attachment?id=18392
Kind unknown from the address
The page says nothing about the size
Nothing has been fetched yet.
timetable.xlsx
Excel workbook
The page says 84 KB
Nothing has been fetched yet.
orders.sqlite
SQLite database
The page says nothing about the size
Nothing has been fetched yet.
course-materials.zip
ZIP archive40 MBFiles 10Folders 2
- Folders
- 2
- Packed
- 40 MB · slides and PDFs barely shrink
- Type
- application/zip
Read 67 KB of 40 MB. The rest never left the server.
Processed locally · Foretaste reads files on your device and does not upload them.
Presentation · 4 files · 21 MBDocument · 4 files · 18 MBSpreadsheet · 1 file · 86 KBText file · 1 file · 3.1 KB
Inside this archive
| Day | Time | Module |
|---|---|---|
| Mon | 09:00 | Statistics I |
| Tue | 11:00 | Research methods |
| Thu | 14:00 | Seminar |
# Course materials Slides are in lectures/, one deck a week. Readings are in readings/ - start with chapter 1. The syllabus says what is assessed and when.
Open a folder, then a file: opening one fetches that file and nothing else. How a ZIP is listed from its end
attachment?id=18392
PDF document412 KBPages 6
- Identified by
- its first bytes, 25 50 44 46 2D = %PDF-
- Type
- application/octet-stream
The whole file was read - 412 KB. The browser's own viewer needs every byte before page one.
Processed locally · Foretaste reads files on your device and does not upload them.
The address had no extension; the first bytes decided. Switch to Foretaste's own viewer and a page costs only its ranges. How a file is identified
timetable.xlsx
Excel workbook84 KBSheets 1Rows 3
The whole file was read - 84 KB. An Office file has no index to read instead.
Processed locally · Foretaste reads files on your device and does not upload them.
| Day | Time | Module | Room |
|---|---|---|---|
| Mon | 09:00 | Statistics I | B204 |
| Tue | 11:00 | Research methods | A112 |
| Thu | 14:00 | Seminar | C009 |
Values only: formulas are not recalculated. Word, Excel and PowerPoint
orders.sqlite
DatabaseTables 3
Read from the header and the schema pages only. Counting rows walks every table, so that is a button.
Processed locally · Foretaste reads files on your device and does not upload them.
- customers 6 columns
- orders 9 columns
- order_items 5 columns
A schema from a few kilobytes at the front of the file. SQLite in more detail
What this shows: the gesture, the panel and the meter. What it costs: nothing — this one is a reconstruction, and the figures in it are measured elsewhere. Close the panel to start again.
Demo 02
What would Foretaste call this file?
Type a filename. The answer comes from the extension's own detection table, read out of its source at build time.
What this shows: the last and weakest of the four rules — a page's own
download and type attributes come first, then the site rules, then
the filename. What it costs: nothing; no request is made. On a real link the bytes
get the final say, which is demo 03.
Demo 03
Your own file, read in this tab
The page has nowhere to send it, which you can check in the network panel.
What this shows: the bytes deciding, against whatever the name claimed. What it costs: nothing leaves this tab — there is no upload and no server call.
Demo 04
A 19 MB archive, listed from a few kilobytes
field-trip.zip lives on this server. Open your browser's network panel before you press the button and watch the range requests arrive.
What this shows: the claim the whole product rests on, measured live rather than asserted. What it costs: about 4 KB to list it, and then one file's own bytes to open one. Reload the page to run it again.
What the extension does that this page cannot
It opens a link on any website. A web page is not allowed to read a file on another site unless that site says so, and almost none do. The only way round that from a page is a server that fetches the file for you, which would mean the file passing through ours. The extension is granted that access by your browser instead, so the file goes from the site it is on to your tab and nowhere else.
It opens far more than this. Word, Excel and PowerPoint drawn on the page, PDFs, databases, 7z and RAR, video and audio, fonts, calendars. This page shows the idea; the format list is what the extension does with it.