Running the Example Media: EKG Strip Image through Simplifier.net's .NET validator the value for data does not pass the regex set for the field type:

[...]

ewout helped me figure out that this is likely because the whitespace after 'WlDQ1' does not come at a 4-char boundary. The regex allows whitespace once every 4 characters, this string fails that constraint.

Potentially this is an issue with the regex itself because we can't find a mention of this in the underlying specification (rfc4648 (ietf.org)).