the file format or file extension is not valid.

Hello, while opening excel file I am getting this Alert and cannot open the files:

Excel cannot open the file 'adj as sk email .xlsx' because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

Microsoft Office help communities

What is the file type and extension.?

Excel files have the extensions:

• xlsx – Excel workbook.
• .xlsm – Excel macro-enabled workbook; same as xlsx but may contain macros and scripts.
• .xltx – Excel template.
• .xltm – Excel macro-enabled template; same as xltx but may contain macros and scripts.

Hmm. This latest information makes it sound like you have been syncing folders between your Mac and iCloud. This would probably be by using the iCloud drive feature as described here - https://support.apple.com/en-gb/HT206985

I would first make a copy of all these files in to a different place - ideally an external drive.

Then I would turn iCloud drive off.

Then I would make sure all remnants are gone.

Then I would setup iCloud drive again and see if it pulls down fresh copies from iCloud.

If it does use them, if it does not copy them from your backup and see if these get pushed up to iCloud.

Having the correct file extension for a particular type of file is very important although I have seen that -

1. Windows users often get them wrong
2. Windows users often mistakenly think changing the file extension also changes the format
3. Windows and Windows apps might be more forgiving than Mac apps

For Excel files a larger number of possible file extensions exist than you might think, once more it is very important to use the correct one so it matches what the file really is.

Excel

Legacy

Legacy filename extensions denote binary Microsoft Excel formats that became outdated with the release of Microsoft Office 2007. Although the latest version of Microsoft Excel can still open them, they are no longer developed. Legacy filename extensions include:

.xls – Legacy Excel worksheets; officially designated "Microsoft Excel 97-2003 Worksheet"

.xlt – Legacy Excel templates; officially designated "Microsoft Excel 97-2003 Template"

.xlm – Legacy Excel macro

OOXML

Office Open XML (OOXML) format was introduced with Microsoft Office 2007 and became the default format of Microsoft Excel ever since. Excel-related file extensions of this format include:

.xlsx – Excel workbook

.xlsm – Excel macro-enabled workbook; same as xlsx but may contain macros and scripts

.xltx – Excel template

.xltm – Excel macro-enabled template; same as xltx but may contain macros and scripts

Other formats

Microsoft Excel uses dedicated file format that are not part of OOXML and use the following extensions:

.xlsb – Excel binary worksheet (BIFF12)

.xla – Excel add-in or macro

.xlam – Excel add-in

.xll – Excel XLL add-in; a form of DLL-based add-in

.xlw – Excel work space; previously known as "workbook"

If you turn on Optimize storage on iCloud Drive, older, little used docs will be moved to cloud if you need the space on your drive.

Sometimes, even Microsoft applications cannot open their own document formats. I use the free LibreOffice v6.2.3 (current) to open these non-corrupted documents, and then save them again. Often this solves the problem, and the respective Microsoft application can then open the document. Keyword is non-corrupted though.

You should be able to just open them in Numbers, though I don't know for sure if it can open an .xls file.

Have you tried that for the one that is corrupted?

Any macros and some formulas will not work in Numbers.

I don't understand why they quit working does it have something to do with the 16bit changes apple is doing?

If you mean when Apple only supports 64 Bit apps in the next version of macOS, no, it has nothing to do with that.

Did you get a copy of the current, free LibreOffice from the link I provided earlier, and open the .xls file with it. If that works, you can save it as .xls or .xlsx, and then it will open again in Excel or Numbers.

Because LibreOffice is obtained from its Developer site, you will have to undergo a one-time ritual to satisfy macOS security. Once you have installed it, visit the Applications folder, right-click on LibreOffice, and just select Open. Apple's Gatekeeper will verify the application with a progress bar, and then you will receive a dialog with another Open button. Click that too.

LibreOffice will start, and subsequent usage is the normal double-click (or single-click in Launchpad, or Dock) to launch it. You will want LibreOffice Calc from the initial application panel, as that is the Excel clone.

I don't understand why they quit working does it have something to do with the 16bit changes apple is doing?

No, not at all.

Why would the files be corrupted?

Nobody but Microsoft could possibly answer that question.

Did you change your AppleID username to the att.net address, or did you create a new AppleID with the ATT.net user name?

For the former, there should be not "path" to change. If you did create a new AppleID, there is no way to merge the accounts. They are forever, entirely separate.