Do not detect validity of har file based on extension

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Create har file without the har extension.
2. Try and analyze it. 
3. Error: Check your uploaded file format: FILENAME.
4. Rename to FILENAME.har and reupload.
5. Har file is analyzed.

What is the expected output? What do you see instead?

I would expect it to detect valid har data inside of the file, not the 
extension of the file.

Original issue reported on code.google.com by bberenb...@gmail.com on 4 Feb 2013 at 7:55

[GoogleCodeExporter]

Hi, pcaphar is expecting the file uploaded to be a pcap file, pcaphar then 
convert the pcap to har and view it, which is the main purpose for this 
project. If there are more interest in auto-detecting har instead pcap, we will 
do it. Thanks.

Original comment by ls...@google.com on 4 Feb 2013 at 8:45