title: excel_sheets function in R gives : Error: Evaluation error: zip file - when reading an excel file which is open in MS EXCEL
body: <p>Why does <code>&lt;b&gt;excel_sheets&lt;/b&gt;</code> function in <code>&lt;b&gt;R::readxl&lt;/b&gt;</code> package - not read from excel files which are open? </p>
  
  <p>I tried searching for anwer on google and in stackoverflow threads but could not find any answers. Closest i get was to understand that this package detects the file as a zip file if the excel file is open. But i couldn't find why.</p>
  
  <pre><code>library(readxl)
  boy.file.names &lt;- list.files("boys",full.names = TRUE)
  ## all files are saved in a folder called boys in my working directory ##
  
  excel_sheets(boy.file.names[[1]])```
  ## if i have this excel file open in MS EXCEL application i get this error ##
  ----------------------------
  Error: Evaluation error: zip file 'boys/~$1996boys_tcm77-254026.xlsx' cannot be opened.
  -----------------------------
  </code></pre>
  
  <p>I expected the file read operation not to be affected by open/close status of the file. But the R package can read the excel file as soon as i close it on MS EXCEL.</p>