The input slot that will be used to access the value.
label
Display label for the control, or NULL for no label.
multiple
Whether the user should be allowed to select and upload
multiple files at once. Does not work on older browsers, including
Internet Explorer 9 and earlier.
accept
A character vector of MIME types; gives the browser a hint of
what kind of files the server is expecting.
width
The width of the input, e.g. '400px', or '100%';
see validateCssUnit.
Details
Whenever a file upload completes, the corresponding input variable is set
to a dataframe. This dataframe contains one row for each selected file, and
the following columns:
name
The filename provided by the web browser. This is
not the path to read to get at the actual data that was uploaded
(see
datapath column).
size
The size of the uploaded data, in
bytes.
type
The MIME type reported by the browser (for example,
text/plain), or empty string if the browser didn't know.
datapath
The path to a temp file that contains the data that was
uploaded. This file may be deleted if the user performs another upload
operation.
providing 
.