refactor: centralize file extension logic in mixin

- Remove .pdf extension from envelopes (they are containers, not files)
- Use metadata.extension when available for accurate file type display
- Extract extension logic to fileExtension computed property in FileEntryMixin
- Eliminate code duplication between FileEntry and FileEntryGrid components

Signed-off-by: Vitor Mattos <1079143+vitormattos@users.noreply.github.com>

Author: vitormattos

src/views/FilesList/FileEntry/FileEntry.vue

@@ -14,7 +14,7 @@
 			<FileEntryPreview :source="source" />
 			<FileEntryName ref="name"
 				:basename="source.name"
-				:extension="'.pdf'" />
+				:extension="fileExtension" />
 		</td>
 
 		<!-- Actions -->

src/views/FilesList/FileEntry/FileEntryGrid.vue

@@ -10,7 +10,7 @@
 			<FileEntryPreview :source="source" />
 			<FileEntryName ref="name"
 				:basename="source.name"
-				:extension="'.pdf'" />
+				:extension="fileExtension" />
 		</td>
 
 		<!-- Checkbox -->

src/views/FilesList/FileEntry/FileEntryMixin.js

@@ -45,6 +45,13 @@ export default {
 				color: `color-mix(in srgb, var(--color-main-text) ${ratio}%, var(--color-text-maxcontrast))`,
 			}
 		},
+
+		fileExtension() {
+			if (this.source.nodeType === 'envelope') {
+				return ''
+			}
+			return this.source.metadata?.extension ? '.' + this.source.metadata.extension : '.pdf'
+		},
 	},
 	methods: {
 		// Open the actions menu on right click