MediaAttachment special cases

[abraham]

Add a MetaDetails entity with the following properties

### `width` {#width}

**Description:** The width of the attachment in pixels.\
**Type:** {{<nullable>}} Integer, or null if the attachment is not visual\
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### `height` {#height}

**Description:** The height of the attachment in pixels.\
**Type:** {{<nullable>}} Integer, or null if the attachment is not visual\
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### `frame_rate` {#frame_rate}

**Description:** The frame rate of the video attachment.\
**Type:** {{<nullable>}} String (fraction), or null if the attachment is not a video\
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### `duration` {#duration}

**Description:** The duration of the video attachment.\
**Type:** {{<nullable>}} Float, or null if the attachment is not video or audio\
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### `bitrate` {#bitrate}

**Description:** The media bitrate of the video or audio attachment.\
**Type:** {{<nullable>}} Integer, or null if the attachment is an image\
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### `aspect` {#aspect}

**Description:** The media aspect ratio of the video or image attachment.\
**Type:** {{<nullable>}} Float, or null if the attachment is not visual\
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Update MediaAttachment#meta to include small and original.

### `meta[small]` {{%optional%}} {#meta-small}

**Description:** Details about the small preview version of the attachment.\
**Type:** {{<nullable>}} [MetaDetails](#MetaDetails) or null\
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### `meta[original]` {{%optional%}} {#meta-original}

**Description:** Details about the original version of the attachment.\
**Type:** {{<nullable>}} [MetaDetails](#MetaDetails) or null\
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Ref mastodon/documentation#1735