File types and file name extensions are often used interchangeably, but they aren’t quite the same thing.
File name extensions are the characters that appear at the end of a file name. Write file name extensions in lowercase, preceded by a period. Don’t write them in all caps.
Examples: .csv, .docx, .xlsx, .jpeg; not .CSV, .DOCX, .XLSX, .JPEG
File type, or file format, refers to a file’s coding. When you’re discussing a file type, or file format, use the formal name of the type. The file type name is often in all caps because many file type names are acronyms or initialisms. Do not use the file name extension to refer generically to the file type.
In the file name mydata.csv, the file name extension is .csv, indicating that this is a CSV file. Changing the file name extension from .csv to .mp3 doesn’t change the file type from CSV to MP3.
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Make sure the file you’re uploading has the .xlsx file name extension, or the upload won’t work.
Make sure the file you’re uploading is an XLSX file, or the upload won’t work.