jodd-mail/src/main/java/jodd/mail/EmailAttachmentBuilder.java
protected String resolveContentType(final String contentType) {
		if (contentType != null) {
			return contentType;
		}
		if (name == null) {
			return MimeTypes.MIME_APPLICATION_OCTET_STREAM;
		}

		final String extension = FileNameUtil.getExtension(name);
		return MimeTypes.getMimeType(extension);
	}
protected String resolveContentType(final String contentType) {
		if (contentType != null) {
			return contentType;
		}
		if (name == null) {
			return MimeTypes.MIME_APPLICATION_OCTET_STREAM;
		}

		final String extension = FileNameUtil.getExtension(name);
		return MimeTypes.getMimeType(extension);
	}
Resolves content type from all data.

@param contentType Content type if we know it. {@code null} is fine to use.
@return content type