When attempting to upload an Excel file to Salesforce using the Excel uploader, there are cases where the upload fails due to some salesforce upload issue. However, the uploader does not display any error message, but the values are getting updated to the object. So the user is not able to see any error message and its making it difficult for the user to know what went wrong.

Using Excel for Mac and the xlsx format on MacOs Catalina with english but german locale the following is observed:

• I receive "Error: unknown" errors when uploading a file with references to fields which hold the native formats for Date, DateTime.
• I also receive wrong values (to big by the magnitude of 10) for fields with the native Percentage format.

Workaround:
In order to upload such files I need to use the TEXT() function in additional cells to turn the native values into formatted string representations, e.g.

• Date: '=Text(DateCell;"Yyyy-MM-DD")'
• DateTime: '=Text(DateTimeCell;"Yyyy-MM-DD hh:mm:ssZ")'
• Percentage: '=Text(PercentageCell;"##%")'

It would be helpful if this could be documented or at least the error would be bit more descriptive.

Comments:

• The used library (SheetJS) seem to be able to derive the formatted values (e.g. https://oss.sheetjs.com/sheetjs/) from the original cells.
• Hence I assume based on the documentation for cell-object that instead of using the "Raw Value" as in 'record[field] = cell.v;' it might be better to read the "formatted value" - cell.w for such data types.