luci-base: ensure that file upload values have length

Ensure that the (table) length of a file upload value has nonzero length
by initializing the first table index with the file name.

This fixes tests in the form

    x = luci.http.formvalue(...)
    if x and #x > 0 then ... end

Fixes #1763.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/luasrc/http.lua

@@ -359,6 +359,7 @@ function mimedecode_message_body(src, msg, file_cb)
 			then
 				field.name = lhttp.header_attribute(buffer, "name")
 				field.file = lhttp.header_attribute(buffer, "filename")
+				field[1] = field.file
 			end
 
 			if field.headers then