Added MIME type image/png for credential attributes

[Dindexx]

Short description of what this resolves:

With this PR, credentials with attributes of MIME type 'image/png' can now be stored and retrieved.

Changes proposed in this pull request:

• Added Support for MIME type image/png.
• Value of attributes with MIME type 'image/png' is the image encoded in base64.
• Added Test for storing credential with attributes of MIME type 'image/png'.

Fixes: #