Add support for custom avatars (via file path) and for not touching the already-set avatar

This is based on the work done in https://github.com/etkecc/baibot/pull/60 by https://github.com/Fmstrat (Ben Curtis),
with various changes on top to make the code more idiomatic and flexible.

This commit squashes the following patches (newest first):

- Improve handling of `user.avatar` configuration (null & empty string being the same now) and add support for a special `keep` value
- Minor import reordering
- Simplify avatar configuration (`user.avatar.source` -> `user.avatar`)
- Combine `logo_bytes` and `mime_type` determination logic and do not fall back to default avatar if reading the custom avatar file fails
- Switch from deprecated `mime_guess::guess_mime_type(avatar_path)` to `mime_guess::from_path(avatar_path).first_or_octet_stream()`
- Relax `mime_guess` constraint and order alphabetically
- Use `mime` from `mxlink`
- Add mime-type support and switch to user.avatar.source
- (Original work by Fmstrat) Add support for custom avatars

Co-authored-by: Fmstrat <nospam@nowsci.com>

src/entity/cfg/config.rs

@@ -1,7 +1,7 @@
 use std::path::PathBuf;
 
 use mxlink::helpers::encryption::EncryptionKey;
-use serde::{Deserialize, Serialize};
+use serde::{Deserialize, Deserializer, Serialize};
 
 use crate::{
     agent::{AgentDefinition, AgentPurpose, PublicIdentifier},
@@ -83,6 +83,52 @@ impl ConfigHomeserver {
     }
 }
 
+/// Configuration for the bot's avatar.
+///
+/// - `Default`: Use the built-in default avatar (null, empty string, or missing in config)
+/// - `Keep`: Don't touch the avatar, keep whatever is already set ("keep" in config)
+/// - `Custom(String)`: Use a custom avatar from the specified file path
+#[derive(Debug, Clone, PartialEq, Serialize)]
+pub enum Avatar {
+    /// Use the built-in default avatar
+    Default,
+    /// Keep the current avatar, don't change it
+    Keep,
+    /// Use a custom avatar from the specified file path
+    Custom(String),
+}
+
+impl Default for Avatar {
+    fn default() -> Self {
+        Avatar::Default
+    }
+}
+
+impl<'de> Deserialize<'de> for Avatar {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let value: Option<String> = Option::deserialize(deserializer)?;
+        Ok(match value {
+            None => Avatar::Default,
+            Some(s) => Avatar::from_string(s),
+        })
+    }
+}
+
+impl Avatar {
+    pub fn from_string(value: String) -> Self {
+        if value.is_empty() {
+            Avatar::Default
+        } else if value.eq_ignore_ascii_case("keep") {
+            Avatar::Keep
+        } else {
+            Avatar::Custom(value)
+        }
+    }
+}
+
 #[derive(Debug, Serialize, Deserialize)]
 pub struct ConfigUser {
     pub mxid_localpart: String,
@@ -93,6 +139,9 @@ pub struct ConfigUser {
 
     #[serde(default)]
     pub encryption: ConfigUserEncryption,
+
+    #[serde(default)]
+    pub avatar: Avatar,
 }
 
 impl ConfigUser {

src/entity/cfg/env.rs

@@ -6,6 +6,7 @@ pub const BAIBOT_HOMESERVER_URL: &str = "BAIBOT_HOMESERVER_URL";
 pub const BAIBOT_USER_MXID_LOCALPART: &str = "BAIBOT_USER_MXID_LOCALPART";
 pub const BAIBOT_USER_PASSWORD: &str = "BAIBOT_USER_PASSWORD";
 pub const BAIBOT_USER_NAME: &str = "BAIBOT_USER_NAME";
+pub const BAIBOT_USER_AVATAR: &str = "BAIBOT_USER_AVATAR";
 pub const BAIBOT_USER_ENCRYPTION_RECOVERY_PASSPHRASE: &str =
     "BAIBOT_USER_ENCRYPTION_RECOVERY_PASSPHRASE";
 pub const BAIBOT_USER_ENCRYPTION_RECOVERY_RESET_ALLOWED: &str =

src/entity/cfg/mod.rs

@@ -2,4 +2,4 @@ mod config;
 pub mod defaults;
 pub mod env;
 
-pub use config::Config;
+pub use config::{Avatar, Config};