A system emailed you report.xls, you double-clicked it, and Excel put up a warning: the file format and extension of 'report.xls' don't match. Open it anyway and it either works or fills the screen with markup. Upload it to the Excel viewer instead — that reads what the file is rather than what it is called. XLS, XLSX, CSV, ODS and the macro, binary and template variants, up to 20 MB, deleted when you close the tab.

An extension is a claim, not a fact

Nothing stops a program from writing any bytes it likes and naming the result .xls, and a great many of them do. Reporting tools, ERPs, banking portals and admin panels all offer "Export to Excel", and the cheapest way to build that feature is to emit something else and give it the Excel extension, because Excel will usually open it.

Since Excel 2007 the file is checked before it opens, which is where the warning comes from. It is not a corruption message. It is Excel telling you the name is wrong.

Finding out what you actually have

Open the file in a text editor — Notepad will do — and look at the first line. Five things account for nearly all of it.

<html, <head> or <table — an HTML table with a spreadsheet name on it. Browsers and web reports produce these constantly.
<?xml — SpreadsheetML, the XML workbook format Excel 2003 wrote.
PK — a ZIP archive, so a real XLSX (or ODS, or DOCX) with the wrong extension.
Unreadable binary starting with the bytes D0 CF 11 E0 — a genuine pre-2007 XLS. This one is what it says it is.
Plain readable text with commas, semicolons or tabs between values — a delimited text file, which is a CSV wearing someone else's name.

What each impostor costs you

The point of knowing is that they lose different things.

An HTML table carries the values and some of the formatting, and no formulas at all. The exporting system worked the numbers out and wrote down the answers. There is nothing underneath to recalculate, and nothing was lost in transit — it was never written.

SpreadsheetML does keep formulas and cell formats. What it cannot carry is charts and pivot tables; the format has no way to store them, so they were dropped when the file was written, not when you opened it.

A misnamed XLSX loses nothing. It is a whole modern workbook and the only thing wrong with it is six characters of filename.

A delimited text file has values and nothing else: no formulas, no formatting, no second sheet. If it looks like one column of text with commas in it, the separator was not the one your copy of Excel expected.

Fixing it, once

For anything you have to keep, open it in Excel past the warning, then File, Save As, Excel Workbook (.xlsx). You now have a real workbook and the warning is gone for good.

For a misnamed XLSX, rename it and stop. Renaming is not converting and almost never helps — this is the one case where it does, because the contents were already correct.

The one that is not an impostor

.xlsb looks suspicious and is not. It is a genuine Excel workbook stored as binary instead of zipped XML: same data, noticeably smaller, and it opens faster on a large model. If a colleague sends you one, nothing is wrong with it.

Whatever the file turned out to be, the Excel viewer will open it and show you the sheets.