Blob Subtypes (API Guide)
Although you manage Blob data in ways similar to other data types, InterBase provides more flexible data typing rules for Blob data. Because there are many native data types that you can define as Blob data, InterBase treats them generically and allows you to define your own data type, known as a subtype. InterBase also provides two predefined subtypes: 0, an unstructured subtype generally applied to binary data or data of an indeterminate type, and 1, applied to plain text.
User-defined subtypes must always be represented as negative integers between –1 and –32,678.
A subtype of a Blob column is specified when the Blob column is defined.
The application is responsible for ensuring that data stored in a Blob column agrees with its subtype; InterBase does not check the type or format of Blob data.