Use Properties|MIME-Type to change the SVN MIME-type of the selected files. The MIME-type can be either a default Text, a default Binary or a Custom type. In case of a Custom type, you have to specify the corresponding MIME-type here. E.g. 'text/html', 'application/pdf' or 'image/jpeg'.
MIME-types can't be arbitrary strings but must be well-formed. For instance, a MIME-type must contain a '/'. By default, SmartSVN checks whether MIME-types are well-formed. Use Force to disable this check.
The MIME-types are relevant for some SVN operations, for instance updating, where in case of text types the line endings, etc. can be replaced. By default, when adding files (see Add), the coarse MIME-type (either text or binary) is automatically determined by SmartSVN. In general this detection is correct, but in certain cases you may want to explicitly change the MIME-type of the file with this command.
Within the project settings you can define file name patterns which should always be treated as binary.