I have registered thumbnail in the following way
<bean id="thumbnailDefinitionSTLpreview" class="org.alfresco.repo.thumbnail.ThumbnailDefinition">
<property name="name" value="stl" />
<property name="mimetype" value="application/sla" />
<property name="transformationOptions">
<!-- The thumbnail definition have transformationOptions mandatory -->
<!-- But there are none for pdf, so use SWFTransformationsOptions as a dummy -->
<bean class="org.alfresco.repo.content.transform.swf.SWFTransformationOptions">
<property name="flashVersion" value="9" />
</bean>
</property>
</bean>
<bean id="baseThumbnailRegistry" abstract="true">
<property name="contentService" ref="ContentService"/>
<property name="renditionService" ref="renditionService" />
<property name="transactionService" ref="TransactionService" />
<property name="tenantAdminService" ref="tenantAdminService" />
<property name="redeployStaticDefsOnStartup" value="${system.thumbnail.redeployStaticDefsOnStartup}" />
<property name="thumbnailDefinitions">
<list>
<ref bean="thumbnailDefinitionMedium" />
<ref bean="thumbnailDefinitionDoclib" />
<ref bean="thumbnailDefinitionImgpreview" />
<ref bean="thumbnailDefinitionAvatar" />
<ref bean="thumbnailDefinitionAvatar32" />
<ref bean="thumbnailDefinitionPDFpreview" />
<ref bean="thumbnailDefinitionWebpreview" />
<ref bean="thumbnailDefinitionSTLpreview" />
</list>
</property>
<property name="thumbnailRenditionConvertor" ref="thumbnailRenditionConvertor" />
</bean>
and in "web-preview.get.config.xml" have also added following code
<condition mimeType="application/sla">
<plugin src="webpreview" paging="true">STLViewer</plugin>
<plugin srcMaxSize="500000">Image</plugin>
</condition>
<condition thumbnail="stl">
<plugin src="stl" paging="true">STLViewer</plugin>
</condition>
and in order to create a thumbnail, I am executing a script on a folder rule which is as follows
document.createThumbnail("stl", true);
document.save();
and also able to see "cm:thumbnail" child in node browser as shown in below image

But unfortunately, I am not getting the preview of files with "application/ply" mime type.
I am attaching two more images below to make you more clear with a situation

this is how my actual file with "application/sla" mime type is getting previewed. But file with mime type is not getting preview properly which can be seen in below image

And If I am performing the transformation from "application/ply" to "application/sla" it is working correctly.