Determine file format
Determine if files are xls or xlsx (or from the xlsx family).
excel_format(guess = TRUE) is used by read_excel() to
determine format. It draws on logic from two lower level functions:
format_from_ext() attempts to determine format from the file extension.
format_from_signature() consults the file signature or "magic
number".
File extensions associated with xlsx vs. xls:
 xlsx: .xlsx, .xlsm, .xltx, .xltm
 xls: .xls
File signatures (in hexadecimal) for xlsx vs xls:
 xlsx: First 4 bytes are 50 4B 03 04
 xls: First 8 bytes are D0 CF 11 E0 A1 B1 1A E1
excel_format(path, guess = TRUE) format_from_ext(path) format_from_signature(path)
| path | Path to the xls/xlsx file. | 
| guess | Logical. If the file extension is absent or not recognized, this controls whether we attempt to guess format based on the file signature or "magic number". | 
Character vector with values "xlsx", "xls", or NA.
files <- c(
  "a.xlsx",
  "b.xls",
  "c.png",
  file.path(R.home("doc"), "html", "logo.jpg"),
  readxl_example("clippy.xlsx"),
  readxl_example("deaths.xls")
)
excel_format(files)Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.