Fileupload

<g:uploadForm> lo único que hace es agregar el atributo enctype="multipart/form-data" al formulario.

GSP:

Preview:
<g:if test="${uploadedFileName}">
    <a href="${grailsApplication.config.apache.docroot.location + "/" + uploadedFileName}" target="_blank">
        <img src="${grailsApplication.config.apache.docroot.location + "/" + uploadedFileName}"
             class="img-thumbnail" width="200" height="230"/>
    </a>
</g:if>

Link to image:
<g:if test="${uploadedFileName}">
    <a href="${grailsApplication.config.apache.docroot.location + "/" + uploadedFileName}" target="_blank"><i
            class="fa fa-file-photo-o fa-2x"></i></a>
</g:if>

<g:uploadForm action="formSubmit">
    <input type="file" name="myFile" />
    <g:submitButton name="submit1" value="enviar"/>
</g:uploadForm>

Controller:

import org.springframework.security.access.annotation.Secured
import org.springframework.web.multipart.commons.CommonsMultipartFile
import us.incorpora.sigrem.utils.Htdocs

@Secured(['permitAll'])
class TestController {

    def index() {
    }

    def formSubmit() {
        CommonsMultipartFile multipartFile = request.getFile('myFile')
        if (multipartFile.isEmpty()) {
            flash.error = "Debe elegir un archivo"
            redirect action: "index"
            return
        }
        
        Htdocs htdocs = new Htdocs(dir: grailsApplication.config.sigrem.uploads.location)
        File file = htdocs.transferMultipartFile(multipartFile, "base_filename")

        render view: "index", model: [uploadedFileName: file.name]
    }
}

Si se desa utilizar el plugin bootstrap-fileinput, solo hay que modificar <input type="file" name="myFile" /> por <t:fileinput name="myFile"/> y agregar en el head <t:assetsFileinput/>.