How to detect a file-upload examining the http-message? - http

Question

I want to detect if a Http-Request is a file-upload or not. I know, there are a view things that may indicate a file-upload:
request method: If its POST, there's usually a message-body. But I know, it's also possible to send content withing a GET-request. Is it possible to upload files with a GET-request aswell?
content-type: I guess, the content-type field is usually set, file-upload-message. But whats the content-type of a file-upload?
content-length: The content-length field should be set for a file-upload.
There are a view Questions left:
How can I distinguish a file-upload from a normal html-form post? Does the browser use chunked-encoding for file-uploads? (As far as I know, that would be senseless, but I dont know a lot)

Answer 1

Usually it can be detected by checking whether the request is multipart .
The following example code is c&p from Apache Commons FileUpload library
/**
* Utility method that determines whether the request contains multipart
* content.
*
* #param request The servlet request to be evaluated. Must be non-null.
*
* #return <code>true</code> if the request is multipart;
* <code>false</code> otherwise.
*/
public static final boolean isMultipartContent(
HttpServletRequest request) {
if (!"post".equals(request.getMethod().toLowerCase())) {
return false;
}
String contentType = request.getContentType();
if (contentType == null) {
return false;
}
if (contentType.toLowerCase().startsWith(MULTIPART)) {
return true;
}
return false;
}
where MULTIPART is
/**
* Part of HTTP content type header.
*/
public static final String MULTIPART = "multipart/";

Answer 2

Checking for a multipart form submission just gets you through the front door. The problem is, you can have a multipart form submission that does not actually contain a file upload. If you want to know if you actually have an uploaded file, you need to search through the form parts. Like this:
public static int getUploadCount(HttpServletRequest request) throws Exception {
int fileCt = 0;
String[] tokens;
String contentDisp;
String fileName;
// Search through the parts for uploaded files
try{
for (Part part : request.getParts()) {
fileName = "";
contentDisp = part.getHeader("content-disposition");
// System.out.println("content-disposition header= "+contentDisp);
tokens = contentDisp.split(";");
for (String token : tokens) {
if (token.trim().startsWith("filename")) {
fileName = token.substring(
token.indexOf("=") + 2,
token.length() - 1
);
}
}
if (!fileName.equals("")) {
fileCt++;
}
}
} catch (ServletException ex) {
throw new Exception(ex);
}
return fileCt;
}