How to validate the file type of a file uploaded by the user in a Shiny app?

I'm trying to create a Shiny App (using R Studio).

I want to use a fileInput widget (named ffile) to read an xlsx or xls file from the user. However, I need it to make sure that it's the correct file type, otherwise the rest of the code will not work. I read about the validate() and need() functions. So I did it like this:

data<-reactive({ 
infile = input$ffile 
if (is.null(infile)) 
return(NULL)
ext<-c('application/vnd.ms-excel','application/vnd.openxmlformats-officedocument.spreadsheetml.sheet')

o<-is.element(infile$type,ext)
validate(need(o,'Wrong ext. Select .xls, .xlsx or .csv'))
file<-read.xlsx(infile$datapath, 1)
return(file) })

I tried loading a .docx document and it was successfully blocked and the warning message was displayed as desired. However, when I try to load a correct .xlsx file it would still show the warning message instead of actually accepting it. I do not know if I'm using the validate/need incorrectly or there is something about MIMEs that I don't quite understand. Help would be appreaciated.

Solution

You can set the accepted MIME directly in the fileInput object in your ui.R using the accept argument:

fileInput('file1', 'Choose CSV File',
                accept=c('application/vnd.ms-excel',
                         'application/vnd.openxmlformats-officedocument.spreadsheetml.sheet',
                         '.xls',
                         '.xlsx'))

This will only let the user select excel files from the file browser window that opens.

In your server.R, you can then just get the data from the file without validating.