Motivation:
According to https://www.ietf.org/rfc/rfc2388.txt 4.4, filename after "content-disposition" is optional and arbitrary (does not need to match a real filename).
Modifications:
This change supports an extra addBodyFileUpload overload to precise the filename (default to File.getName). If empty or null this argument should be ignored during encoding.
Result:
- A backward-compatible addBodyFileUpload(String, File, String, boolean) to use file.getName() as filename.
- A new addBodyFileUpload(String, String, File, String, boolean) overload to precise filename
- Couple of tests for the empty use case
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