disable preview on file upload dropzones

The docs for react-dropzone say that we have to explicitly
free the 'file preview' to avoid memory leaks in the browser (!).
Presumably disabling previews means we don't have to manage this
ourselves.

client/views/Exams.js

@@ -186,7 +186,11 @@ class Exams extends React.Component {
               <h5 className='subtitle'>then we know that to do with PDF's</h5>
 
               <Dropzone accept=".yml, text/yaml, text/x-yaml, application/yaml, application/x-yaml"
-                style={{}} activeStyle={{borderStyle: 'dashed', width: 'fit-content', margin: 'auto'}} onDrop={this.onDropYAML}>
+                style={{}} activeStyle={{borderStyle: 'dashed', width: 'fit-content', margin: 'auto'}}
+                onDrop={this.onDropYAML}
+                disablePreview="true"
+                multiple="false"
+                >
                 <div className="file has-name is-boxed is-centered">
                   <label className="file-label"> 
                     <span className="file-cta">
@@ -235,8 +239,11 @@ class Exams extends React.Component {
 
               <Dropzone accept={"application/pdf"} style={{}}
                 activeStyle={{borderStyle: 'dashed', width: 'fit-content', margin: 'auto'}}
-                onDrop={this.onDropPDF.bind(this)}
-                disabled={isDisabled}>
+                onDrop={this.onDropPDF}
+                disabled={isDisabled}
+                disablePreview="true"
+                multiple="true"
+                >
                 <div className="file has-name is-boxed is-centered">
                   <label className="file-label">
                     <span className="file-cta" disabled={isDisabled}>