bugfix: fix multipart panic + support for filename + content-type

[Ice3man543]

Proposed changes

Able to fuzz multipart files requests like below now with correct propagation of files content-type and filename.

-----------------------------13382841176870751782402671908
Content-Disposition: form-data; name="surname"

test
-----------------------------13382841176870751782402671908
Content-Disposition: form-data; name="image"; filename="xss.svg"
Content-Type: image/svg+xml

<!DOCTYPE ccabca [ <!ENTITY ccabca SYSTEM "file:////etc/passwd"> ]><x>&ccabca;</x>

Checklist

• Pull request is created against the dev branch
• All checks passed (lint, unit/integration/regression tests etc.) with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)