If you already have the data in a String or a Stream, you don't need a
DataSource to build your MimeBodyPart.  You can go straight to the
MimeMultipart object and add the type and content directly to it.

The DataSource stuff is just a convenience for doing that with data from
a file.  Something like this perhaps:

String pdf_data
MimeMultipart content = new MimeMultipart("alternative");
MimeBodyPart pdf = new MimeBodyPart();
pdf.setContent(pdf_data, "application/pdf");
content.addBodyPart(pdf);

Presumably FileDataSource implements some interface possibly called
DataSource. You don't want a FileDataSource. You want some other
flavour, perhaps something you concoct yourself from scratch the
implments the interface.

One way is not to use FileDataSource but to implement your own
DataSource.  Something like this (here I have byte[] to store the
data, but you may have it as String or any other storage that is
appropriate in your case):

public class MyDataSource implements javax.activation.DataSource {

    private byte[] data        = new byte[0];
    private String contentType = "text/plain";
    private String name        = null;

    public void setData(byte[] data) {
        this.data = data;
    }

    public void setContentType(String contentType) {
        this.contentType = contentType;
    }

    public void setName(String name) {
        this.name = name;
    }

    public InputStream getInputStream() {
        return new ByteArrayInputStream(data);
    }

    public OutputStream getOutputStream() {
        return null;
    }

    public String getContentType() {
        return contentType;
    }

    public String getName() {
        return name;
    }

}