diff --git a/locales/en-US.yml b/locales/en-US.yml
index 413d097f6..36a733b94 100644
--- a/locales/en-US.yml
+++ b/locales/en-US.yml
@@ -710,6 +710,8 @@ botProtection: "Bot Protection"
 instanceBlocking: "Blocked Instances"
 selectAccount: "Select account"
 switchAccount: "Switch account"
+cannotAttachFileWhenAccountSwitched: "You cannot attach a file while switched to another account."
+cannotSwitchAccountWhenFileAttached: "You cannot switch accounts while files are attached."
 enabled: "Enabled"
 disabled: "Disabled"
 quickAction: "Quick actions"
diff --git a/packages/client/src/components/post-form.vue b/packages/client/src/components/post-form.vue
index 3d5434096..04b7d3800 100644
--- a/packages/client/src/components/post-form.vue
+++ b/packages/client/src/components/post-form.vue
@@ -7,7 +7,12 @@
 >
 	<header>
 		<button v-if="!fixed" class="cancel _button" @click="cancel"><i class="fas fa-times"></i></button>
-		<button v-tooltip="i18n.ts.switchAccount" class="account _button" @click="openAccountMenu">
+		<button
+			v-tooltip="files.length > 0 ? i18n.ts.cannotSwitchAccountWhenFileAttached : i18n.ts.switchAccount"
+			:disabled="files.length > 0"
+			class="account _button"
+			@click="openAccountMenu"
+		>
 			<MkAvatar :user="postAccount ?? $i" class="avatar"/>
 		</button>
 		<div>
@@ -45,7 +50,14 @@
 		<XPollEditor v-if="poll" v-model="poll" @destroyed="poll = null"/>
 		<XNotePreview v-if="showPreview" class="preview" :text="text"/>
 		<footer>
-			<button v-tooltip="i18n.ts.attachFile" class="_button" @click="chooseFileFrom"><i class="fas fa-photo-video"></i></button>
+			<button
+				v-tooltip="postAccount != null ? i18n.ts.cannotAttachFileWhenAccountSwitched : i18n.ts.attachFile"
+				:disabled="postAccount != null"
+				class="_button"
+				@click="chooseFileFrom"
+			>
+				<i class="fas fa-photo-video"></i>
+			</button>
 			<button v-tooltip="i18n.ts.poll" class="_button" :class="{ active: poll }" @click="togglePoll"><i class="fas fa-poll-h"></i></button>
 			<button v-tooltip="i18n.ts.useCw" class="_button" :class="{ active: useCw }" @click="useCw = !useCw"><i class="fas fa-eye-slash"></i></button>
 			<button v-tooltip="i18n.ts.mention" class="_button" @click="insertMention"><i class="fas fa-at"></i></button>